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/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_id;
 	struct wl_list unmanaged_surfaces;
 
 	struct seat seat;