xwayland: keep the xdg_toplevel around

The xdg_toplevel object was used solely when creating the window
surface, and the value of the object discarded.

To be able to make the surface fullscreen using the xdg_toplevel
protocol, we need to have access that object, so keep it around along
with the xwl_window.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>