Reorganized public API a bit in in ETModelElementDescription, ETPackageDescription and ETEntityDescription (plus documented some API that were not)

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/EtoileFoundation@10298 7b3f36aa-e6db-49c1-8b4c-66eac1997e64