add window urgency through xdg-activation-v1

urgency is done through activation requests without a serial from a
previous interaction.

https://gitlab.freedesktop.org/wayland/wayland-protocols/-/issues/150

niri-visual-tests/src/test_window.rs

@@ -272,4 +272,8 @@ impl LayoutElement for TestWindow {
     fn interactive_resize_data(&self) -> Option<InteractiveResizeData> {
         None
     }
+
+    fn is_urgent(&self) -> bool {
+        false
+    }
 }