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.
Map Analysis mode: selection color is now used when rendering affected things.
Fixed, Map Analysis mode, "Check unused things" check: checker returned false-positive results when used in Doom map format.
Map Analysis, internal: some ErrorCheckers and ErrorResult had inconsistent namespaces.
Thing Edit forms, internal: optimized missing flags check a bit.
