Memento(Redo|Undo)Command has a noop for the undo or redo respectively, and
we don't need both before and after state. This is primarily useful for
drag start/finish callbacks, and really only makes sense where wrapped by
(begin|commit)_reversible_command (a composite command).
Also a few more "normal" MementoCommands.
git-svn-id: svn://localhost/ardour2/branches/undo@695 d708f5d6-7413-0410-9779-e7cbd77b26cf
8e301e875a