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.

gtk/gtkwidgetprivate.h

@@ -152,6 +152,12 @@ gchar **          _gtk_widget_list_action_prefixes         (GtkWidget    *widget
 GActionGroup *    _gtk_widget_get_action_group             (GtkWidget    *widget,
                                                             const gchar  *prefix);
 
+void              _gtk_widget_add_controller               (GtkWidget           *widget,
+                                                            GtkEventController  *controller,
+                                                            GtkPropagationPhase  phase);
+void              _gtk_widget_remove_controller            (GtkWidget           *widget,
+                                                            GtkEventController  *controller);
+
 G_END_DECLS
 
 #endif /* __GTK_WIDGET_PRIVATE_H__ */