Rename ModelBroker to ObjectBroker in preparation to make it more generic.
Move some of the signal monitoring code here.
Add basic bidirectional signal communication over the network.

This is for the few remaining places that split client and server not
along a model/view boundary.
Expand the object broker to support NetworkObject instances as well.
Improve naming.
Client-side part of NetworkObject.
Server-side part of NetworkObject.
Test integration of NetworkObject for performing method calls.
Properly handle proxy models when synchronizing selection models.

Now hierarchies and client-side sorting do no longer mess up the selection.
Listen to selection rather than current changes.

Only selections are reliably mapped between client and server.