For attributed string merge, allow -resolveConflictsFavoringSourceIdentifier: to control the order of conflicting inserts (e.g. insert 'x' and index 0 and insert 'y' at index 0)

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10081 7b3f36aa-e6db-49c1-8b4c-66eac1997e64