more flexible connectors, link stairs to bridges, bugfix

fix: do not unsnap snappy edges when adding a tube or adding or removing a track

fix bug with pedestrian edges unsnapping from road network when updating the station

bugfix with auto bridges

more freedom placing stairs and underpasses + small optical fix for roofs

small visual improvements

small fix for auto boards on mixed stations + extra cargo capacity

two new underground stations

new and / or prettier underground stations

tweak bounding boxes