Duplicating document between wikis may cause objects to be broken

The reason for this to be critical, is that it breaks the creation of subwikis in XEM based on an existing wiki.

While duplicating objects of a document, the reference is xClass reference is duplicated absolute, and does not change to the new wiki. This had no immediate impact, since object of a class from a external wiki is not supported. But the reference is later used by the local uid serializer and cause it to not remove the wiki from the class in the referenced object, which therefore change the generated id.

The consequence are that the properties of the object are stored with an inappropriate id, and could never be retrieve later.

IMO, we should prevent the store from storing such object, since the store does not support object with the class of an external wiki. We should also ensure to not render local reference absolute when the store load them.