Incremental reference updates

[markus1978]

When an object is moved within the containment hierarchy its fragment based URI changes and we therefore update the object's entry in the extrinsic ID index. But, for the objects contained in the containment hierarchy of the moved object (at least within the same fragment) the fragment based URI changes also.

When an object is moved, we need to iterate through the contents of that object within the same fragment and change all objects fragment based URI in the extrinsic ID index.

This is similar to #12

[markus1978]

Resolved with #29