This new function is callable explicitly, unlike the old function that
used to have the same name.
This will be needed when tearing down what
weston_desktop_xwayland_init() puts up.
Since calling weston_desktop_client_destroy() for an external client
(one that has a wl_resource for this) is a bug, add asserts to prevent
it. This will only be needed for the internal client: XWM.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>