Add documentation for QtCallback
Rename QtCallbackProxy to QtSignalForwarder

This name gives a better indication of what the class does.

Also rename connectCallback() to connect() for brevity.  This
does mean that we're overloading QObject::connect() though
which could be confusing.  May reconsider in future.
Rename disconnectCallbacks() to disconnect() for consistency
Update the README following the QtCallbackProxy -> QtSignalForwarder class rename