QtRemoteObject source or host should have notification of replica's being active

There currently doesn't seem to be a way to know on the source side of a remote object connection what the status of the connection(s) is. That's strange. There is a heartbeat mechanism, but there doesn't appear to be a way a monitor it.

It would be useful to get notification of remote replicas being added and removed. That API should probably be on the QRemoteObjectHost object, but I would also support it being on the source objects themselves or through an event send to the source objects.