popover: Track toplevel focus changes

popover: Track toplevel focus changes

Make the popover temporarily undo the GTK+ grab, so it remains modal
to its window, but does not attempt to steal focus on other non-modal
windows that get the focus.

This was most confusing with keyboard navigation, as the focus would
remain stuck on the popover, and not move to the newly focused window
after the popover was dismissed. It didn't have as much effect on
pointer operations as only the first click would be consumed in order
to hide the popover.