provide an easier way to add mixed (audio+MIDI) tracks to a session, to facilitate...

provide an easier way to add mixed (audio+MIDI) tracks to a session, to facilitate the use of plugins like Reaktor which accept both audio & MIDI data. there's probably still a paradigm shift waiting to happen here but its likely post-3.0.

git-svn-id: svn://localhost/ardour2/branches/3.0@12816 d708f5d6-7413-0410-9779-e7cbd77b26cf