add setting/resetting of wall tile types
Similar to adding objects and tiles the user can now also set/reset the wall types (read: wall papers) on either side of a piece of wall.
This is slightly more complicated as in the case of tiles, as here both sides of a piece of wall have to be regarded individually.
This change had effects on the shaders, the renderer, the World class, the SelectionState, the GameState and some other crowd favourites.
What is, however, still missing though is the removal and placement of walls.