Added, Visual mode, GLDEFS, GLOOME: subtractive glow is now supported.

Changed, Visual mode: changed thing fog calculation logic. Should be closer to GZDoom now.
Fixed, GLDEFS parser: "height" texture parameter was not treated as optional.
Fixed, text lump parsers: in some cases incorrect line number was displayed in error and warning messages.
Fixed, Visual mode: glow effect was not applied to sectors with 3 sidedefs.
Fixed, Visual mode: in some cases glow effect was not updated when replacing textures. 
Fixed, general interface: "Full Brightness" button state was not updated during map loading.
Fixed, Drag Linedefs/Vertices/Sectors/Things modes: positions of line length labels were not updated while panning the view.
Cosmetic: added a bunch of new icons.
Cosmetic: changed Visual mode crosshair.