When we create a new session and are using a template from an old version of
Ardour, we should not issue the VersionMismatch dialog and not make a copy of
the session file for the old version.
We need to extend the signature of Session::load_state() to tell it if we are
creating a session from a template. Session::_is_new cannot be used for it
because it has a the semantics if to auto connect the the master bus.
687b3078c3