This is in particular relevant for the ::is-focus property, because
updating that one doesn't cause enter/leave events.
But it also checks that notify and enter/leave happen in the right
order.
Split the focus tracking into a separate
GtkEventControllerFocus, and change the API one more time.
We are back to having ::focus-in and ::focus-out signals.
Update all users.
Check that we get the expected sequences of focus
change events for the nonlinear, inferior and ancestor
cases.
It would be nice to do the same checks for crossing
events, but we have no gtk_window_set_hover().