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/view.h

@@ -174,6 +174,7 @@ struct view {
 
 	bool mapped;
 	bool been_mapped;
+	uint64_t creation_iid;
 	enum lab_ssd_mode ssd_mode;
 	enum ssd_preference ssd_preference;
 	bool shaded;