Sound Propagation Mode: fixed a problem where removed sector geometry was still shown when undoing/redoing. Fixes #992

Sound Propagation Mode: fixed a crash when things were outside the map's geometry bounding box. Fixes #975

In Sound Propagation Mode you can set the start (default: Shift+S) and end (default: Shift+E) sectors between which a path the sound can travel will be found and displayed.

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

Call RedrawSurface in OnRedrawDisplay of NodesViewerMode, SoundPropagationMode and SoundEnvironmentMode so that the map gets updated properly and not being stretched/squished when the windows is resized

Added, Find & Replace mode, UDMF: added Linedef activation flags to the "Find Linedef flags" search mode flags list.
Changed, Sound Propagation mode: all sound zones are now shown when no sector is highlighted.
Changed, Sound Environments mode: the mode is now available only in UDMF map format.
Changed, Color Picker plugin: the plugin functionality is no longer available in Doom map format.
Restored the ability to create superimposed lines by dragging them with "Snap to Geometry" mode disabled.
Fixed, Sound Propagation mode: fixed a crash when a single-sided linedef had "Block Sound" flag.
Fixed, Find & Replace mode: in some cases "Find Sector/Sidedef/Linedef/Thing flags" search modes failed to find map elements with required flags.
Fixed, Edit Selection mode: in some cases incorrect geometry was created after applying multipart sector edit when "Replace with Dragged Geometry" mode was enabled.
Fixed a crash caused by eventual GDI font objects overflow.

Added, Sound Environments mode: sound environments of the same type are now colored using the same color.
Added, Sound Environments mode: current sound environment is now highlighted (can be toggled using "Toggle Highlight" action).
Added: Tag selectors now have up/down buttons.
Fixed, Sound Environments mode: sound environments were not updated after performing Undo/Redo actions.
Fixed, Sound Propagation mode: sound zones were not updated after performing Undo/Redo actions.
Internal: moved "Toggle Highlight" action to the core, also changed it's category to "View".
Internal: "Toggle Highlight" action state is now saved in the Program configuration.
Updated ZDoom_DECORATE.cfg (GetZAt).
Updated ZDoom_linedefs.cfg (Sector_SetPortal args).

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 :)

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.

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.

Changed, Sound Environments mode: previously selected sound environment is now automatically selected when inserting a new sound environment item. Also it's now impossible to accept the form without selecting a sound environment.
Changed, Sound Propagation mode: single-sided lines with sound zone boundary flag are now highlightable and clickable.

Added: nested thing categories can now be defined in DECORATE using "//$Category" special comment. Syntax is the same as in SLADE 3.
Added: thing categories defined in Game Configurations can now be nested.
Changed, Thing Edit window: thing categories now use different icons.
Fixed: Thing Edit and Vertex edit windows had incorrect help links.
Fixed: Sound Propagation and Sound Environment modes had incorrect help links.
Documentation: updated "DECORATE keys" and "Things Settings" pages.

Changes since 5bf51d0:
Large scale cleanup & refactoring.
Sound Propagation Mode: you can now click on doublesided linedefs to toggle sound blocking flag.
Sound Environment Mode: you can now click on doublesided linedefs to toggle sound zone boundary flag.
Sound Propagation Mode: reimplemented the highlighting of non-deaf things. Things are now highlighted using selection color.
Removed "Show tooltips" checkbox. Tooltips are now always enabled.
Fixed, Sound Environment Mode: thing tooltip about multiple active sound environment things was shown even when there was only one active sound environment thing.
Fixed, Sound Environment Mode: "Configure colors" toolbar button was not working because of missing action.
Fixed, cosmetic: "Configure colors" toolbar icon was missing a tooltip.
Fixed, cosmetic: display was not redrawn after changing colors using "Color configuration" window.