COObjectGraphContext: change any APIs that returned sets of COObject to return sets of ETUUID, or array of COObject. See -[TestBranch testOverriddenIsEqualsObject] for the motivation

git-svn-id: svn://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@9988 7b3f36aa-e6db-49c1-8b4c-66eac1997e64