window-switcher: add age order to window cycling

- Add a new configuration option to control the window switcher
  traversal order.

- Document cycle windows behavior with new option `order` in
  `<windowSwitcher>`

include/labwc.h

@@ -188,6 +188,7 @@ struct server {
 	struct wl_listener xdg_toplevel_icon_set_icon;
 
 	struct wl_list views;
+	uint64_t next_view_creation_iid;
 	struct wl_list unmanaged_surfaces;
 
 	struct seat seat;