COUndoTrack: migrate -undoNode:,-redoNode: to use the new -addToStoreTransaction: method in COCommandGroup instead of -applyToContext:, and remove -applyToContext: because it is no longer used. This eliminates two parallel implementations of the complex logic in COCommandSetCurrentVersionForBranch.

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10581 7b3f36aa-e6db-49c1-8b4c-66eac1997e64