first pass at the big rethink of managing sort order keys for editor and mixer. this...

first pass at the big rethink of managing sort order keys for editor and mixer. this appears to work, but remote control IDs are not yet correct (frequently off by one because of the presence of the master bus in the editor)

git-svn-id: svn://localhost/ardour2/branches/3.0@12953 d708f5d6-7413-0410-9779-e7cbd77b26cf