ivi-shell: fix cleanup of desktop surfaces

The ivi-shell keeps track of its surfaces by adding them to the ivi_surface_list
to be able to remove them on shutdown. It also creates an ivi_layout_surface for
a desktop surface, but does not keep track of these surfaces.

During compositor shutdown, libweston prints the following message:

	BUG: finalizing a layer with views still on it.

Fix it by adding the created ivi_layout_surface to the ivi_surface_list to
remove the surfaces from the layer during shutdown.

Furthermore, remove the ivi_layout_surface from the desktop surface and free it
when the desktop surface is destroyed.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>