Linedef Edit Form, Sector Edit Form: when several map elements with different textures are selected, the "texture stack" icon is shown in the texture selectors instead of the "missing texture" icon.
Error Checker mode: added "Check missing flats" check.
Error Checker mode: fixed several descriptions. 
Sprite patches for sprites defined in TEXTURES are now loaded from Sprites namespace.
Some of my older texture alignment code didn't work properly when strict texture/flat handling ("mixtextureflats" game configuration flag set to false) was enabled.
Fixed some incorrect checks for missing texture ("-").