COObject: make entity descriptions immutable after they are used by a COObject instance. Tests/Core/TestCOObjectSynthesizedAccessors.m: Remove part of -testSynthesizedAccessorsRestrictedToDynamicProperties that adds a property description to an existing entity, since this is no longer allowed. TestObjectUpdate: tweak the test so it doesn't try to add a new property description to COObject

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10248 7b3f36aa-e6db-49c1-8b4c-66eac1997e64