Update from early tubes with bugs...

... to tubes with less bugs and more features based on feedback.

This includes
 - Fixes for problems and missing functionality found by Nodrak while it was a proposed addition to /TG/
 - Features and improvements mentioned by Summoner and Cael on IRC
 - Partial fix for issue #2577 (You shouldn't be able to drag pods around to begin with, at least not while they are in tubes) (moving them around afterwards, including manually pushing them back into the tube system is still a future feature consideration)

Ideas not included (because they haven't been coded yet):
 - Pods not in tubes being draggable
 - While dragging a tubeless pod, click a tube with an open end to insert the pod
 - An endstop/reverser tube section, with one entrance, which turns entering pods around
 - Construction of tubes