Add a set of new models for crossings to make some new track
arrangements possible. This commit adds perpendicular crossigns that can
be rotated at any angle: 30/45/60/90 and 45/90 degree crossings in two
rotations. In future, further crossing types can be added to the blender
file.
Add the blender file in rail_crossings.blend that is the source of these
files and which refers to the texture inside advtrains_train_track.
Add a set of prototypes for rail crossings to advtrains/tracks.lua and
register the new crossings in advtrains_train_track.
