Don't instantiate an app object in +mainRepository. This doesn't play well in case some code wants to instantiate the app object later (based on a custom subclass).

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10548 7b3f36aa-e6db-49c1-8b4c-66eac1997e64