COObject: Add an 'editingContext' property for API symmetry with COObjectGraphContext, COPersistentRoot, and COBranch. Also marked these branch/persistentRoot/objectGraphContext/editingContext properties as weak as a reminder that they are all parents of COObject.

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10149 7b3f36aa-e6db-49c1-8b4c-66eac1997e64
CODiffManager: add -description method

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10150 7b3f36aa-e6db-49c1-8b4c-66eac1997e64
ProjectDemo: update TODO

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10151 7b3f36aa-e6db-49c1-8b4c-66eac1997e64