Manually squash Widgy's first two migrations

This pair of migrations created and then removed a bunch of stuff in
Widgy that depended on Mezzanine. Removing each create/delete pair
allows migrating Widgy without Mezzanine.

Reviewed-by: Rocky Meza <rocky@fusionbox.com>
Reviewed-by: Antoine Catton <acatton@fusionbox.com>
Drop target collision detection

Instead of using the pointer_event_catcher to activate drop targets,
activate the drop target closest to the mouse that overlaps the dragged
element.

As an optimization, only drop targets visible in the viewport are
checked for overlap. The list of visible drop targets is refreshed on
scroll.