Make Stateful::*instant_xml methods protected and non-virtual

Make Stateful::*instant_xml methods protected and non-virtual

Add add_instant_xml and instant_xml methods to the Session class which call
the corresponding Stateful methods with the session directory for the
directory argument

Add add_instant_xml and instant_xml methods to the Configuration class which
call the Stateful methods with ARDOUR::get_user_ardour_path() for the directory
argument

git-svn-id: svn://localhost/ardour2/trunk@2012 d708f5d6-7413-0410-9779-e7cbd77b26cf