implement pushing of jellies

I wrote a much more complicated patch to do this, but Anatoly Vorobey
<avorobey@gmail.com> sent me a much simpler implementation before I had pushed
mine!  This patch is his algorithm with a bug fixed.
fix an animation glitch

If a push led to a fall, the fall could join before the fall animation
completed.
dynamically fill level picker
Merge https://github.com/martine/jelly

 commit 9a28e68bd09858c47ebd452cbba26235095e8787.