Implement "Save/Load State" Feature

[Aklakan]

It must be possible to serialize the application state as a JSON object, from which the state can be later restored.

Although Facete 1 provides code for this purpose, this code needs to be enhanced such that multiple references to the same object can be correctly restored. (The original code is only suitable for tree structures)

[Aklakan]

Removed Conti label - as maybe the issue is not that relevant: Datasets can now be persisted, and in many cases restoring facets manually can be easily done.

As for the approach of solving a part of the issue once and for all: The serializer has been extended to cope with object graph structures, however, the FacetConfig class is not self contained (it references the default constraint factory registry) which prevents meaningful serialization.

Yet, we also need to have the workspace system, and then see again how far we get with the serializer.