Undo: add an alternate API (currently unused) to COCommand which translates them into COStoreActions. I think this may be the way to go to improve performance of applying a lot of undos/redos at once, because it will let us commit a whole batch of undos in a single store transaction.

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10322 7b3f36aa-e6db-49c1-8b4c-66eac1997e64