Add a resource creation signal

This change allows to add a resource creation listener to a wl_client,
which will be notified when a new resource is created for that client.
The alternative would be to have a per wl_display listener, but i think
that resources are really client specific objects, so it makes sense
to use the wl_client as the context.

Signed-off-by: Giulio Camuffo <giulio.camuffo@kdab.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
[Pekka: added wl_list_remove() in TEST(new_resource).]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>