Allow to serialize a DocumentReference with a locale so that it can be parsed back

We don't have any serializer which allows to serialize a DocumentReference with a locale, and so right now we need to record 2 informations, the reference and the locale when we want to store a full document reference in an XObject page property.

Note that apparently the serialization of PageReference with locale is supported by using the syntax documented in https://extensions.xwiki.org/xwiki/bin/view/Extension/Model%20Module#HPageReferences