xdg-shell: Add support for popup repositioning

This adds support for xdg-shell v3, which adds support for repositioning
popups. This adds support for explicit popup repositioning, as weston
doesn't yet apply any constraining to popups, thus cannot implicitly
reposition.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Tested-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>