Fix the broken clients design.

Currently clients are not connected immediately because the user needs to be notified and take action upon the connection request. This makes the design of the RfbClient class a bit strange.

This new approach applies tp-qt4's PendingOperation design to RfbClients.

New connections create a PendingRfbClient that internally handles the user notification and the RfbClient object is only created when the user has accepted the connection.