We now rely on toplevels receiving and forwarding all the events
the windowing should be able to handle. Event masks are no longer a
way to determine whether an event is deliverable ot a widget.
Events will always be delivered in the three captured/target/bubbled
phases, widgets can now just attach GtkEventControllers and let those
handle the events.
Event controllers now auto-attach, and the GtkCapturePhase only determines
when are events dispatched, but all controllers are managed by the widget wrt
grabs.
All callers have been updated.
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.
