Add private GtkWidget api for action groups

Similar to the just-added action muxer api for enumerating
action prefixes and for getting the action group for a
prefix.

https://bugzilla.gnome.org/show_bug.cgi?id=730095

gtk/gtkwidgetprivate.h

@@ -148,6 +148,9 @@ void              _gtk_widget_style_context_invalidated    (GtkWidget    *widget
 
 void              _gtk_widget_update_parent_muxer          (GtkWidget    *widget);
 GtkActionMuxer *  _gtk_widget_get_action_muxer             (GtkWidget    *widget);
+gchar **          _gtk_widget_list_action_prefixes         (GtkWidget    *widget);
+GActionGroup *    _gtk_widget_get_action_group             (GtkWidget    *widget,
+                                                            const gchar  *prefix);
 
 G_END_DECLS