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: 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: 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.
