change implementation of layer display menu items and handlers

The menu items no longer ever apply to the selection, because there will be an Editor-level
action to change things "globally"

gtk2_ardour/route_time_axis.h

@@ -291,6 +291,7 @@ protected:
 	UnderlayMirrorList _underlay_mirrors;
 
 	bool _ignore_set_layer_display;
+	void layer_display_menu_change (Gtk::MenuItem* item);
 
 protected:
 	void update_pan_track_visibility ();