Filter out unsupported types before serialization of item data.

This is rather ugly and not exactly efficient, but there is no other way
apart from trying to serialize and see if it works. QMetaType::save at
least tells us it failed, QVariant::save (which is used by operator<<) just
asserts.
Rename ModelBroker to ObjectBroker in preparation to make it more generic.
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.