Sectors Mode: fixed a problem where dragging a single highlighted sector did not move the things in the sector when synchronized things editing was enabled

(Text labels are removed because of performance issues)

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

Rewritten vertex/linedef/sector dragging logic. Now it processes line-line intersections. And handles more corner cases. And has less bugs. Probably.
Fixed, Edit Selection mode: texture scale was flipped when "Floor/Ceiling Transform" -> "Scale" was enabled.
Changed: a warning is no longer displayed when trying to compile an empty SCRIPTS lump.
Updated ZDoom_DECORATE.cfg.

Fixed, Drag Linedefs, Drag Vertices, Edit Selection modes: in some cases sidedefs were incorrectly removed when modifying a closed section of a multi-part sector.
Fixed, Drag Linedefs, Drag Vertices, Drag Sectors, Edit Selection modes: in some cases sidedef textures were not adjusted after applying the modes, leaving middle textures on double-sided lines.

Added, Edit Selection mode: Added "Adjust height" setting. It can be used to adjust floor/ceiling height of selected sectors based on the sector selection was in and the sector selection was moved into.
Removed "Adjust heights to match relatively with surrounding sector" option from Preferences -> Pasting, because sector height adjustments are now handled by Edit Selection mode.
Fixed, Sector info panel: in some cases 0 deg. floor/ceiling texture rotation was triggering texture offset/scale/rotation UI parts to be shown.
Fixed even more cases when sidedefs belonging to linedefs, which were moved on top of existing linedefs, were incorrectly reassigned when applying Edit Selection and Drag Geometry modes.
Fixed, Bridge mode: in some cases calculated floor/ceiling heights were not applied to the sectors created by the mode.
Changed, internal: changed program's CurrentCulture to InvariantCulture.

Added: sectors are now split into multiple sectors when their shape is divided into several closed shapes by drawing new geometry or dragging existing geometry using Edit Selection and Drag Geometry modes.
Fixed: in some cases sidedefs belonging to linedefs, which were moved on top of existing linedefs, were incorrectly reassigned when applying Edit Selection and Drag Geometry modes.
Fixed: Tag Explorer update fix from R2630 was accidentally placed in an UDMF-only if-block.
Updated ZDoom_DECORATE.cfg (A_WeaponOffset).
Updated documentation ("(G)ZDoom text lumps support" page).

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").

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.

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 :)

Changed, "Select Similar", "Paste Properties Special" actions properties windows, Linedefs: removed "Sidedef Properties" from Linedef flags tab.
Fixed several issues with "Synchronized Things Selection" setting:
- Things were not deselected after dragging a single sector.
- Things were not deselected after editing properties of a single sector.
- Things sector references were not updated after using "Join Sectors" and "Merge Sectors" actions.
- Things (de)selection is now applied to all things, instead of only the ones visible using current Things Filter.

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 emergency map backup system (EMBS). When the editor crashes, it will try to export a backup copy of the map structures. When the same map is loaded again, it will ask the user if he wants to restore the map.
High-DPI: fixed many issues with incorrect controls size (Tag selector width, Comment Editor size, icons size, tabs size etc.).
Cosmetic: changed map geometry dragging undo messages to more descriptive ones.
Fixed, Textures Browser, cosmetic: "Show textures in subdirectories" checkbox should be repositioned when "Long texture names" one is invisible.
Updated ZDoom_DECORATE.cfg (added A_SetFloatSpeed + a couple of fixes).

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.

Fixed, Texture Browser Form: well, I broke "Tab" key functionality again (in previous commit)...
Maintenance: changed curly braces style to match DB2 one (hopefully not breaking anything in the process...).
Maintenance: changed private method names casing to match DB2 one.

Sectors mode: things inside of selected sectors can be moved when dragging sectors. This behavior can be toggled by the new button on Sectors mode's toolbar.
Sectors mode: changed Alt-key handling. Now it only enables "snap to grid increment" mode.
Things mode: "snap to grid increment" mode now works in this mode.
Updated documentation.

Game configurations: added "fixedrotation" property to dynamic lights, which use the angle property as a parameter.
Maintenance: removed a ton of redundant stuff.

Error checks: added "Check unused textures" option.
Replaced MissingTexture3D and UnknownTexture3D.
Sectors mode: restored "Lower/Raise Floor/Ceiling by 8 mp" actions.
Visual mode: in some cases sidedefs were rendered as selected when they were not.
Existing linedefs were not split while drawing new lines in some cases.
Texture and height overrides were not applied correctly in some cases.
Preferences form: "Ctrl+Alt+ScrollUp" and "Ctrl+Alt+ScrollDown" dropdown items were setting the shortcut to "Ctrl+Shift+ScrollUp" and "Ctrl+Shift+ScrollDown".

Added hints for Drag Geometry modes.

- Progressive undo/redo system (much faster and up to 1000 undo/redo levels) Please note that several plugin functions and properties have changed.
- Some small optimizations in the classic 2D rendering (cached linedef flags, reduced some unneeded redrawing)