Visual and performance improvements

Visual and performance improvements:
- More vivid colours for thumbnailed files (drawn with texture replace
mode, which disables all shadowing etc.)
- Active directories also drawn with texture replace mode (makes them
look icy cool :-)
- Got rid of colouring textured nodes (directories) - the shadows are
back! :-) Also Colouring textured nodes was not very efficient.
- STILL TO DO: Completely remove colouring nodes - use for example
textures instead. This is needed to get rid of GL.Material calls, which
are very slow.