gtk: Rework pointer cursor selection

gtk: Rework pointer cursor selection

Check the grab widget (both explicit and implicit) and check for a cursor
from the target widget up to this grab widget. If the target widget is
outside the grab widget, only the grab wigdet's cursor will be checked.

This also means that we have to ensure the cursor is updated on button
releases, as an implicit grab being deactivated must trigger a cursor
lookup from the target widget.