fix metering when monitoring track input. (ran twice)
possible fix for crash on exit - err on the side of caution
fix/workaround window-proxy session-management

The window proxy defers construction of classes.
set_session() is called before the instances are available.
the proxy-manager only calls set_session() for SessionHandlePtr
but not the class-specific method..

fixes http://tracker.ardour.org/view.php?id=5566