ETEntityDescription: cache -allPropertyDescriptionNames, it's called a lot by CoreObject. Also add a FIXME related to caching

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/EtoileFoundation@10414 7b3f36aa-e6db-49c1-8b4c-66eac1997e64