All the obvious MementoCommand grunt work. Now there's some
add_undo/add_redo_no_execute sprinkled around where one is separated from the
other (e.g. in different callbacks) or perhaps even where there's only an undo
and no redo. Also some sigc-based undo/redo pairs that probably need their own
Command class.
git-svn-id: svn://localhost/ardour2/branches/undo@692 d708f5d6-7413-0410-9779-e7cbd77b26cf
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