Implement 'under' child transition animation, use it for 'over' for right and down directions, matching 'over' description.

Edit: Also clip bottom child.

Edit2: Also fix crossfade, leading to cleaner code for 'under', and add the actual 'under' child transition, which is a reverse of 'over'.

Fixes #79 (closed), #84 (closed)

(with a CSS snippet to make backgrounds opaque so that they don't overlap)