WIP: own handling for ball<-->border

This is work-in-progress and feels "broken" in some situations. 

• In particular, the old problem of hitting hidden planes seems back (I have to see how to address this _directly_ this time).
• Also the left/right spin is sometimes crazy and sometimes not working at all (not sure what that causes, actually).

Generally feels like regression sometimes, but technically the own collision handling for ball<-->band gives _much_ better control and potential for other stuff. So: meh and yeah at the same time.