gesture: Simplify gesture/widget interaction public API

gesture: Simplify gesture/widget interaction public API

The propagation phase property/methods in GtkEventController are gone,
This is now set directly on the GtkWidget add/remove controller API,
which has been made private.

The only public bit now are the new functions gtk_gesture_attach() and
gtk_gesture_detach() that will use the private API underneath.

All callers have been updated.