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)

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

Editing modes: fixed some crashes related to undoing/redoing while objects with associations were highlighted

Improved associations and event lines. Refactorings to make the whole thing more sensible (hopefully). Options to display labels on event lines (can be configured in Preferences -> Editing).

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

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

* Mouse selection threshold

* Cleanup

* Fix selection bug

Fixed: inheritance from game configuration for dynamic lights; Fixed: visual representation of inherited lights (reported by Nash); Fixed: inherited thing arguments from game configuration; Added: //$ClearArgs for clearing inherited arguments.

* A direct tag-to-tag relationship can now be established between two things, which allows event lines to be properly setup between them. This is intended for interpolation and patrol specials which are linked via a shared tag, instead of an argument. A POSITIVE value represents the parent thing, where a NEGATIVE value represents the child thing.
* Added interpolation and patrol point specials to the persistent event line processing

Updated, Textures Browser: texture name length is now taken into account when calculating the width of texture items.
Internal: moved thing helper shapes creation logic from ThingsMode to LinksCollector.

Added, Things mode, Visual mode: added the ability to specify minimum/maximum argument range hint shapes using special DECORATE comments and Game Configuration properties (loosely based on range finder patch by Xabis).
Updated documentation ("Game Configuration - Action Argument Settings" and "DECORATE keys" pages).

Fixed, Script Editor: MD5 hash of text files stored in PK3 was calculated incorrectly, resulting in false warning messages when saving.

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.

Fixed, Things mode: fixed a crash when deleting Light/Ambient Sound things when "Show Dynamic Light Radii" / "Show Ambient Sound Radii" settings were enabled.
Fixed: WAD lump index was not appended to lump name when reporting errors during ZDoom text lumps parsing.

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

Added, Things mode: sound radii are now shown for "Custom Ambient Sound" and "Custom Ambient Sound (no gravity)" things.

Added, Things mode: minimum/maximum sound radii can now be displayed for ZDoom's Ambient Sound things.
Added, Things mode: added "Show Light Radii" and "Show Sound Radii" main toolbar and View menu buttons.
Fixed, Action arguments control: invisible splitter between args1-3 and args4-5 is no longer draggable.
Updated ZDoom ACC (SetTranslation).
Updated ZDoom_ACS.cfg (SetTranslation).

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, Things mode: thing selection labels must be updated when performing undo/redo.
Fixed another case of invalid sectors being created after dragging map elements when using "Merge Dragged Geometry" and "Replace with Dragged Geometry" drag modes.

Merged "Toggle models, dynamic lights and fog rendering" and "Toggle Geometry Effects" actions into "Toggle Enhanced Rendering Effects" action.
Removed "Dynamic light size" and "Dynamic light intensity" settings.
Updated documentation ("Rendering toolbar" page).

Added, DECORATE: added "//$ArgNRenderStyle" special comment. It allows to specify a rendering shape to display the value of given argument. Supported values are "circle" and "rectangle".
Added, DECORATE: added "//$ArgNRenderColor" special comment. It allows to specify a color used by the rendering shape.
Updated documentation.

Added, Visual mode: added support for FLATSPRITE, ROLLSPRITE, WALLSPRITE and DONTFLIP sprite rendering flags. Removed the older GLOOME-specific implementation.
Changed, "Align Things to Nearest Linedef" action: the action can now also align things with FLATSPRITE and WALLSPRITE flags (previously only things with attached models were aligned).
Changed, "Align Things to Nearest Linedef" action: the action can now align things to the middle part of double-sided linedef, if it has a middle texture assigned. 

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).

Changed: "Unable to save the map: there are too many unique sidedefs!" error message now displays info about sidedefs counts before and after sidedef compression.
Fixed, Draw Grid mode: in some cases invalid geometry was created when drawing very large grids.
Fixed, Things mode, "Rotate Clockwise" action: fixed inability to change thing angle from 0 to 315 when "doomthingrotationangles" Game Configuration property was set to true.
Updated ZDoom_DECORATE.cfg.

Added "Split Joined Sectors" Edit menu item and toolbar button. When enabled, joined sectors adjacent to drawn lines will be split.
Added "doomthingrotationangles" Game Configuration property. When enabled, editor actions related to changing thing angle will snap the resulting angle to 45 degree increments. This property is set to true for vanilla game configurations.
Fixed a crash when changing game configuration from one without Thing actions support to one with them while in Things mode.
Fixed, cosmetic, DB2 bug: current editing mode button was deselected after reloading resources.
Updated documentation ("Game Configuration - Basic Settings" page).

Fixed, Things mode: fixed a crash when switching to Things mode when more than 256 things were selected.

Added: holding Control key while switching to/from Visual mode will now temporarily disable position synchronization (works only when 'Synchronize camera position between 2D and 3D modes' Preferences option is enabled).
Fixed severe lock-up after finishing dragging many vertices/linedefs/sectors in Classic modes.
Updated documentation ("Synchronizing camera position").

Updated ZDoom_DECORATE.cfg (ALLOWTHRUFLAGS flag).

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, Sectors, Linedefs, Things modes: map element selection order of rectangular selection is now direction-dependent.