Listen for notify::sequence-state-changed on the controller, so the
only way to manipulate a sequence state are gtk_gesture_set_sequence_state()
and gtk_gesture_set_state().

Also, make use of gesture groups, so the sequence state is set at once
on all gestures pertaining to a single group. Within a widget, if a sequence
is claimed on one group, it is made to be denied on every other group.