Separate station definitions from model files

Just like we did for the ships, the definitions are now loaded from data/stations. After this it's also possible to use the new style models for (unanimated) stations.

Two questions:
- SpaceStationType.cpp implements _get_string, _get_int... lua getters again, got a nicer place for these?
- Each station type sets two global functions, stationname_ship_dock_anim and stationname_ship_approach_waypoints - maybe you'll want something nicer for this