Add the ability to remotely access methods and properties of QAbstractItemModel subclass objects

If an object of type derived from QAbstractItemModel is remoted then the its additional methods and properties cannot be accessed via the acquired QAbstractItemModelReplica.

The ability to access these in the same way as a dynamically remoted / QRemoteObjectDynamicReplica object would be very useful.

Remoting the object by both methods to get the functionality of both replica types does not work as then calling disableRemoting on the object on the host node does not disable both. 

Trying to then call enableRemoting on both results in one of the enables failing with the warning 'qt.remoteobjects: Tried to register QRemoteObjectSource twice'.