Coding for undo/redo starts in earnest. Paul and I decided to go with a
standard gang of four Command pattern, with serialization. This overcomes the
terrible difficulties we were having with static type checking and the sigc++
approach. I'm adding the requirement that each command support undo,
simplifying undo/redo. NOTE that an important fallout here is that
Command::operator()() is the opposite of the old UndoAction::operator()(), i.e.
Command::operator()() is execute/redo, and Command::undo() is undo.
This commit is a reworking of the infrastructure, and won't compile until
creating Command subclasses for the various commands being performed. That is
primarily where you find get_memento and/or calls to add_(undo|redo.*).
git-svn-id: svn://localhost/ardour2/branches/undo@655 d708f5d6-7413-0410-9779-e7cbd77b26cf
