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.