Improve object persistence

[s-heppner]

At the moment, we do not have a good way of persisting objects, other than writing them to XML or JSON.
In the future, it might make sense to extend the ObjectStore concept with new subclasses to allow it to retrieve objects from arbitrary (but defined) backends, for example a "LocalFileObjectStore, or a MongoDBObjectStore`.
When we do that, we should also decide for a good module structure.