Introduce FloatableElement mixin

Extracted from FloatingMenuSelectWidget. For use in DateInputWidget
and CalendarWidget (see linked bug), and maybe PopupWidget could use
it too?

This code requires a zen-like concentration from the maintainer to
read, much less modify. I'm not sure if it's because it sucks or
because the problem we're solving is just difficult. I am, however,
pretty sure I failed to achieve it and gotten plenty of things wrong,
but I dare not poke it further, as long the $overlay dialog demo works.

Between this and ClippableElement, I honestly couldn't convincingly
explain how they interact and why this even works. I think we're
measuring, positioning and clipping too many things too many times,
or maybe not enough times and it breaks in some corner cases.

Bug: T112676
Change-Id: I658012b7b4fe398c29dd361912d9fee68d95db21