model: Refereable._set_id_short should change the id_short entry in the Namespace's _backend dict
Currently we only check for duplicate id_shorts in the same Namespace, when an id_short is changed (#100). However, we must also update the NamespaceSet with the new id_short of the Referable object to ensure correct duplicate checking and id_short-resolving in the future.
This must also be implemented for other unique attributes in the future (see #121).
issue