Added autosaving of the current map. Autosaving happens in intervals when the map is changed. It will not overwrite the current map, but rather create new files, just like backups. Autosaving interval and number of files can be configured in the "Recovery" tab of the preferences. Autosaving can also be disabled there (not recommended). Autosaves will not have their nodes built for performance reason.

Fixed a problem where not selected geometry and things could not be dragged when the "don't move selection if any part of it is outside of map boundary" option was enabled

(Text labels are removed because of performance issues)

Added action to change the index of linedefs, things, sectors, and vertices. Can be accessed through a shortcut (not bound by default) or the menu of the respective mode (#903)

Fixed a regression where editing map elements did not preserve the selection order, but used the index order instead. Fixes #791

- editing a highlighted, not selected map element will deselect other selected map elements, then start editing the highlighted map element
- dragging a highlighted, not selected map element will deselect other selected map elements, then start dragging the highlighted map element
- dragging a single map element will keep its selection state. I.e. dragging a selected map element will not deselect it after dragging anymore
  - dragging a highlighted, not selected thing will not select it after dragging
- dragging a highlighted, not selected sector, while other sectors are selected, will only drag the highlighted sector, and not the selected sector (and deselect the selected sectors)

* Extended rendering options for drawing vertices
* Fixed vertex scale slider behaviour
* Force plot highlighted vertex in DragSectorsMode
* Moved ShouldRenderVertices property into the properties region
* made config file variable lower case to be in line with other config file variables

Snap selection to grid: fixed a problem that could cause a crash if geometry got removed by snapping it to the grid

Added map scripting capabilities to UDB (#656). Documentation at https://biwa.github.io/udbscript-docs/

Linedefs Mode: fixed a problem where after splitting a linedef the insert vertex preview would not work correctly
Vertices Mode: fixed a problem where after inserting a vertext the insert vertex preview would not work correctly

Linedefs Mode, Sectors Mode, Vertices Mode, Sound Environment Mode, Sound Propagation Mode: slightly improved performance when moving the mouse

Things Mode: performing the smart grid transform action will now set the grid origin to the selected or highlighted thing
Sectors Mode: performing the smart grid transform action will now reset the grid transform

Vertices Mode: the preview vertex is now correctly shown on intersections between the closest line and the transformed grid

Added option to lock texture offsets of 3D floors when dragging the tagged sectors

Moved smart grid transform into Linedefs Mode and Vertices Mode, so it also works when simply highlighting a linedef or vertex. When doing smart grid transform on a linedef it'll also use the vertex closest to the mouse cursor as the origin instead of always the start vertex

- Added option to enable additive paint selecting without holding shift independently from regular additive selecting

* Mouse selection threshold

* Cleanup

* Fix selection bug

Added, Errors and Warnings window: most of warnings/errors related to (G)ZDoom text lump parsing can now be double-clicked to show warning/error source in the Script Editor.
Fixed: some map format conversion errors were never displayed, because errors list was cleared after performing map conversion.
Fixed: 3d floor indicators in 2D modes were not updated after leaving Visual mode.
Fixed: some UDMF-only interface buttons were never removed after converting map format from UDMF.

Updated ZDoom_DECORATE.cfg.
Updated documentation ("Game Configuration - Sectors Settings" page).

Changed, Vertices/Linedefs/Sectors/Things modes: selected map element is no longer deselected after accepting Edit window when it was the only selected map element when the Edit window was opening.

Fixed: parts of UniFileds.UniValuesMatch() logic were inverted. In some cases this resulted in incorrect behavior of "Select Similar" action in UDMF map format.
Fixed, UDMF: alpha should be ignored when applying sector fade/light color gradients.
Ported eevee's fix for incorrect sector geometry occasionally created after dragging map elements (more info: https://github.com/sirjuddington/SLADE/issues/542).

Added, Drag Vertices\Linedefs\Sectors\Edit Selection modes: sidedefs facing outside of current selection are now reattached/added/removed after moving the selection.
Currently this will work as intended only if the selection ends up either completely inside a single sector or completely outside of any sector. Also this logic won't be applied if the selection contains the lines with only start or end vertex selected.
Internal: some TextLabel refactoring.

Added, Draw Lines mode: additional guidelines and horizontal/vertical line lengths can now be displayed for currently drawn line using "Show guidelines" top menu button.
Changed, Draw Lines mode: line angles are now shown only when "Show guidelines" mode is enabled.
Fixed, Draw Lines mode: in some cases "Snap to cardinal directions" mode was snapping only to diagonals.
Fixed, Draw Lines mode: snap to geometry behaved incorrectly when "Snap to cardinal directions" mode was enabled.
Changed, Things mode: dynamic light shape is now drawn using highlight color when a dynamic light thing is highlighted.
Added more sanity checks to MODELDEFS parser.

Added, Linedefs mode, Things mode: selection numbers can now be displayed and can be toggled using the "View Selection Numbering" mode toolbar button.
Added, Linedefs mode, Things mode: sector tags can now be displayed and can be toggled using the "View Sector Tags" mode toolbar button.
Fixed, Classic modes, DB2 bug: in some cases text label size was queried before it was calculated, resulting in text labels being rendered when they shouldn't.
Updated ZDoom_DECORATE.cfg (A_LogFloat).

Changed, Texture Browser window: "All" textures group is now saved/reselected like the rest of the texture groups when closing/opening the window.
Changed, Classic modes: bigger Thing arrows are now rendered when thing sprite rendering is skipped.
Changed, Classic modes: when "Fixed Things Scale" option is enabled, thing size stays at 2x scale instead of 1x when extra bounding box is rendered.
Added Preferences -> Appearance -> "Things transparency (Things mode)" slider.
Renamed Preferences -> Appearance -> "Things transparency" to "Things transparency (other modes)".
Externalized thing bounding box and arrow texture, used to render things in Classic modes (Textures/ThingTexture2D.png).
Updated ZDoom_DECORATE.cfg (A_SetUserVarFloat, A_SetUserArrayFloat).

Added Preferences -> Appearance -> "Things transparency" slider. It controls Things transparency in all classic modes except Things mode.
Added Preferences -> Appearance -> "Hidden Things transparency" slider. It controls transparency of Things hidden by Things filter in Things mode, of all things :)

Internal: removed some duplicate casts, marked some classes is IDisposable, some streams were disposed more than once.

Fixed, Map Analysis mode, "Check stuck things" check: rewritten parts of the flags checking logic to allow more accurate flag checks.
Fixed, Map Analysis mode: fixed a crash when trying to dissolve an invalid sector when one of it's linedefs referenced it on the both sides.
Fixed, Sectors mode: fixed incorrect undo description when deleting sectors.
Internal: joined declaration and assignment of some more variables.

Sectors mode: removed "Move Things in Selected Sectors" toolbar button.
Sectors mode: added "Synchronized Things Editing" menu item, action and toolbar button. When enabled, things dragging will be synchronized to sector dragging. Regular, Paint and Rectangular selection will also select things (holding Alt while selecting inverts this behavior). Deleting sectors will also delete selected things.
Linedefs mode: added "Synchronized Things Editing" menu item, action and toolbar button. When enabled, selected things will be dragged when dragging linedefs. Rectangular selection will also select things (holding Alt while selecting inverts this behavior).
Changed: selection info now always displays info about all types of map elements.
Fixed: .dbs containing only current map settings was created when trying to save unchanged map. 
Fixed, Visual mode, "Copy Texture" action: when "Use long texture names" option is disabled, texture name as stored in Sidedef should be used, not the 8-char texture name.
Updated documentation.

Added "makedoorceil" game configuration property. Works the same way as "makedoortrack" and "makedoordoor", but for ceilings of door sectors. 
Changed, Game configurations: the editor no longer tries to load DECORATE/MODELDEF/VOXELDEF/GLDEFS/REVERBS lumps when "decorategames" setting is not specified / is set to empty string.
Changed, General interface: "Tools -> Reload MODELDEF/VOXELDEF" and "Tools -> Reload GLDEFS" menu items are no longer shown when current game configuration doesn't support DECORATE. 
Fixed a crash when pasting linedef/thing properties in Hexen map format.
Fixed, Visual mode: Visual Thing resources were not fully unloaded when resetting D3D device leading to crash when switching to the editor from a DX-using game engine (like ZDoom) running in fullscreen.
Fixed: in some cases, when current game configuration supported multiple script compilers, it was possible to open/create a map or change map options without selecting any script compiler.
Fixed, New Map Options window: default map name was not updated when switching game configurations.
Fixed: copied map element properties were not reset after switching to another map.
Fixed: stored textures for "Make Door" action were not reset after switching to another map.
Fixed, Game Configurations window: currently selected test engine name was not updated when pasting test engines from another configuration. 
Fixed, Game Configurations: all "Heretic in Doom map format" configurations were using Doom sector effects list.
Fixed, Game Configurations: all "Strife in Doom map format" configurations were using Doom sector effects list.

Added "Paste Properties Special" actions in "Classic" and "Visual" categories. They work the same way as "Paste Special" action.
Added: "Copy Properties", "Paste Properties" and "Paste Properties Special" options are now shown in the Edit menu if current classic mode supports them.
Changed, Paste Properties Special window: only options relevant to current map format are now displayed.
Changed, Paste Properties Special window, UDMF: all UI-managed options are now available.
Fixed: MAPINFO parser was unable to process "include" directives.
Fixed, General interface: selection info was reset to "Nothing selected" after few seconds regardless of current selection.
Fixed, Visual mode: thing bounding boxes were not updated when changing things positions using Randomize mode.
Fixed, Visual mode: event lines were displayed at incorrect height when entering Visual mode for the first time.
Fixed, Texture Browser window: when MixTexturesFlats Game Configuration option is disabled, textures/flats are no longer shown in the Used group when flats/textures with the same names are used in the map. 
Fixed(?): probably fixed an exception some users reported when trying to initialize a Classic mode after switching from Visual mode with "Sync cameras" option enabled.
Changed, Game configurations, Thing Categories: a block must have at least one thing category property to be recognized as a thing category.
Changed, Visplane Explorer: the plugin now outputs more info when it fails to initialize vpo.dll.
Cosmetic, Thing Edit window, Doom/Hexen map format: adjusted UI layout so thing flags control no longer displays scrollbars in Hexen map format.
Internal: merged methods from UDMFTools into UniFields, removed UDMFTools. 
Updated Inno Setup script (added VC++ 2008 SP1 distributive). 
Updated ZDoom_DECORATE.cfg (A_CheckBlock).
Updated documentation (added "System Requirements" page).

Changed, Drag Linedefs/Vertices/Sectors/Things modes: line length labels are now displayed the same way as in Draw modes.
Changed, Drag Linedefs/Vertices/Sectors/Things modes: "lock movement to cardinal directions" mode (Alt-Shift-Drag) now locks movement in 4 directions instead of 8 and doesn't snap map elements to nearest grid intersections when they are not aligned to it. 
Added, Visual mode, GZDoom, DECORATE: FORCEXYBILLBOARD flag is now supported.
Added, Visual mode, GLOOME, DECORATE: FLOORSPRITE, CEILSPRITE, WALLSPRITE, ROLLSPRITE and STICKTOPLANE flags are now supported (implementation is somewhat broken ATM and probably doesn't work the same way as in GLOOME, because Windows build with most these features is nowhere to be found...).
Fixed, Visual mode: in some cases Thing brightness was calculated incorrectly.
Updated ZDoom_DECORATE.cfg.

Classic modes: added "Toggle Grid" action (default key is Alt-G), new menu item in the View menu and new toolbar button.
Changed, Things mode, cosmetic: tweaked thing sprite/bounding box appearance a bit more...
Fixed, Linedefs mode, cosmetic: fixed event lines and sector highlights flickering when view panning was started while displaying action highlight.

Changed, Things mode: things are now rendered with a bit of transparency, so things beneath other things are now visible.
Fixed, Things mode: in some cases it was impossible to highlight a thing beneath other thing when their positions were the same.
Changed, 2D modes: changed thing bounding box and arrow image.
Fixed, cosmetic, 2D modes: in some cases thing arrows were rendered bigger than thing bounding boxes.