n-level undo should be serializer-neutral

[rockfordlhotka]

Right now, the n-level undo code in UndoableBase has some direct dependencies on MobileFormatter.

That should be changed to rely on the configured serializer.

[JasonBock]

FWIW this issue is how I found it. Basically, during BeginEdit(), the configured formatter is given a BusinessRules instance, but since my formatter doesn't see it implement the custom interface I think it should, it failed.

I added code to fall back to MobileFormatter in this case, but this really shouldn't be needed. I wonder if this issue would be the key to rectifying this, at least partially.