Fix a bug in add/delete listener registration for instantiable
(one-to-many) relations. Listener registration would fail if
the entry associated with the relation did not exist (this is
the case when the parent managed object has only just been
created). The solution is to wait for the "relation" entry
to be created and, when it is added, automatically register
the add/delete listener then. This automatic registration is
managed by a "delayed" config add listener which is registered
against the parent managed object.