window-switcher: add order parameter to allow stable window list ordering

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

`order="focus"` cycling is convenient for quick toggling, but some users - me as well -
prefer a stable taskbar-like order which can now be achieved with `order="age"`.

include/view.h

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