Refactor Undo/Redo

The undo/redo code is not particularly clean or well-written. It is also not API-compatible with the apptools Undo/Redo system.

A proper re-write might mean bringing apptools Undo/Redo into Pyface (at least the core, but possibly all of it, so it can be used there) and then making the TraitsUI implement or adapt to the API for its undo/redo, so that it can integrate with application-level undo/redo.

While I suspect that this code is not heavily used externally to TraitsUI, there may need to take some care not to break the API exposed to editors and handlers without a clear migration path.