Add reimport functionality

[SomaZ]

Add vertex color export and import. This will help with new iterations of maps when mapping.

Paint a map, save color data, later import new iteration of map, import color data.
Color data will be stored based on position. Multiple precision level will be stored, so that moved vertices still find a match.

[SomaZ]

Just add a reimport button instead. Need to keep altered uvs and vertex colors. Also don't reload shaders when doing so. This should help when working on new maps as it eliminates the need to rework things again and again.

[SomaZ]

Changed my mind. I will add a checkbox in the import dialog for reimporting maps. This will delete all bsp mesh models in blender, add changes to the bsp info node and skips material parsing. Like that you won't loose material edits like emissive changes or transparent overrides. Edits to the bsp itself will be lost, but I assume people will use this when they recompiled maps. So edits would be lost nevertheless.