[PATCH] xdg_shell: Add a new shell protocol.

[Rob Bradford]

If we're talking changes...

Let's make the popup return an object with a done event on rather than
stick the event in the interface like popup_done is.

I'd also like to see us standardise on the naming of the request to
create the new object - I think create_xdg_surface is better than
get_xdg_surface and aligns with the create_surface request on
wl_compositor.

Rob