Updated stairwell textures & BSP, added whitebox

Stairwell textures now have 3 parts: bottom, middle and top (to avoid
stretching/seams)

Also completely removed the stairwell and re-subtracted it so the walls
actually line up properly. The stairs will be made as a mesh instead of
with BSP brushes (BSP stairs suck!)

Added a simple whiteboxing cube for fleshing out obstructions in the
level. Placed one of these at the corner of the hallway where some kind
of obstruction will be (fallen in ceiling hopefully)