Previously it used Block, which is less efficient for large numbers of items.
Also move the top margin out of the menu item to fix the baseline alignment of
the text.

Fixes #1615

This patch sizes the menu such that it is always on screen, but doesn't scroll
the menu to ensure that the currently selected item is always visible and on
top of the button. That will need to wait for a later patch.

Also, teach CustomPaint how to repaint animations more efficiently.

Fixes #3720