Add subdirectory support under custom texture category to Dash RED for organization

[GooberRF]

Low priority, but the level editing workflow would be significantly improved if some sort of categorization was possible for custom textures. I think the most straightforward way to do this would be to read .tga/.vbm files from subdirectories under user_maps\textures, and display them in the texture browser as listed below.

As an example, if subdirectories named "QuakeWalls", "TextureSet1", and "MetalBeams" exist within user_maps\textures when RED launches, RED could display them as:
Custom
Custom - QuakeWalls
Custom - TextureSet1
Custom - MetalBeams

Where "Custom" lists .tga/.vbm files in the root of user_maps\textures, and the others list .tga.vbm files present in their corresponding subdirectories. This is similar to the stock RED behaviour for walls, floors, ceilings, and signs. Take floors for example, red.cfg lists 4 categories for floor textures, each in a subdirectory of data\maps\textures\floor. (ie. data\maps\textures\floor\cement, data\maps\textures\floor\metal, etc.)