desktop-shell: Fail if get_xdg_surface is called on an xdg_surface

desktop-shell: Fail if get_xdg_surface is called on an xdg_surface

If a client calls xdg_shell.get_xdg_surface on a surface that is already
an xdg_surface would, prior to this patch, succeed, but cause weston to
crash later when trying to configure. This patch instead sends a role
error to the client complaining that it already is an xdg_surface.

Note that .._set_role() only fails when changing roles, not when setting
the same role twice.

The same is done for xdg_popup.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>