Work towards removal of Session's Diskstream list.
Havn't managed to completely remove it because of Session loading from XML - the
Diskstreams are separate from the Tracks (I assume as a throwback to when they
were distinct) so the Diskstreams need to be stored somewhere until the Tracks
are loaded. Ideally tracks should completely own their Diskstreams - not sure
how to accomplish this without breaking Session loading though...
git-svn-id: svn://localhost/ardour2/branches/midi@687
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