grabHelper: Use Clutter.grab() underneath

grabHelper: Use Clutter.grab() underneath

This is subject to further possible simplifications. Use Clutter.grab
to redirect input and focus, a fundamental difference here is that
we do redirect input to the topmost owner of the grabhelper stack,
instead of the stage. This is better behaved with the presence of
other grabs, at the cost of some behavioral changes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2045>