COObject: enable true type-checking of values against the property description. e.g. it is illegal now to set an NSNumber value for an NSString property

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10251 7b3f36aa-e6db-49c1-8b4c-66eac1997e64