Added visual indicator to Y-shaped transit tubes

A small arrow shows direction preference.
Also added flipped versions.
Ladder tube for N-S and E-W. Can be walked over.

 + supporting code for the ladder tubes
 + pods automatically start moving in all tube types, rather than just stations
Automatic tube state, tries to connect two tubes.
Less precise, but faster to lay out when mapping.
Quick cleanup/fix
Merge pull request #67 from Uristqwerty/master

More tube stuff