(SessionPlaylists). Main point being to ensure that, when a Session
is destroyed (especially as a result of its constructor throwing an
exception) SessionPlaylists::track is disconnected from its signal
at the same time as the list of playlists is destroyed. Otherwise
problems abound.
git-svn-id: svn://localhost/ardour2/branches/3.0@6270 d708f5d6-7413-0410-9779-e7cbd77b26cf