fix for most (all? we can dream ...) issues involved in #4399 (editing multiply-appli...

fix for most (all? we can dream ...) issues involved in #4399 (editing multiply-applies operations to playlists used more than once), and as a side-issue, fix playlist selection which broke when we hid Diskstreams inside Tracks by using orig_track_id() rather than orig_diskstream_id()

git-svn-id: svn://localhost/ardour2/branches/3.0@10968 d708f5d6-7413-0410-9779-e7cbd77b26cf