Allow xwayland views to become urgent when on a non-visible workspace

This removes the urgency stuff from the commit handler and puts it in a
new set_hints handler instead. This allows the xwayland surface to
become urgent without having to commit (which doesn't happen if it's on
an non-visible workspace).

include/sway/tree/view.h

@@ -140,6 +140,7 @@ struct sway_xwayland_view {
 	struct wl_listener set_title;
 	struct wl_listener set_class;
 	struct wl_listener set_window_type;
+	struct wl_listener set_hints;
 	struct wl_listener map;
 	struct wl_listener unmap;
 	struct wl_listener destroy;