COUndoTrack: add a customRevisionMetadata property for providing extra metadata for the track to attach to commits (e.g. ProjectDemo username metadata)

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10629 7b3f36aa-e6db-49c1-8b4c-66eac1997e64