Expose app-id for Wayland applications

For use in Budgie Desktop, and perhaps XFCE itself down the road, it would be very nice to be able to get the `desktop-id` from an Application. On X11, there isn't a great way to do this, but there is with Wayland, as each surface should have the desktop-id set on it (assuming the xdg_shell protocol is implemented). It looks like the XfwApplicationWayland class already stores this as a property, it just isn't exposed anywhere.