Cleaning up ActiveSession logic in vtkSMProxyManager. The way it was implemented
resulted in us missing session change signals on occassion. That was resulting
in hacks that were spread throughout the code. Cleaned that up.

Also added a new vtkSMSession::Disconnect() method that UI and Python can use to
disconnect from a session gracefully. I'd like to move towards making
pqServer::removeServer() pretty much obsolete and simply use
vtkSMSession::Disconnect(). It almost does that now, except there are a few
signals it fires that need to be tracked down.

These changes make it possible for the Python modules (servermanager and simple)
to live on in a Python interpreter even after disconnects. This is required
since after commit cd547dbd, we no longer
destroy the Python (sub) interpreter on disconnect.

Change-Id: I9a2b25177cb14c09a1d75105757c69de99ce6f72