Enable multiple MirSurfaceItems rendering the same MirSurface
So MirSurface is the model and MirSurfaceItem is the view+controller.
+ Make MirSurfaceItem instantiable from QML + Isolate all mir::scene::Surface code inside MirSurface + Enhanced qml-demo-shell: you can now move, resize and *clone* windows Approved by: Gerry Boland