Implement the EditWeight undo command. Since there is common code
(storage of the old weight), this creates a common base class for
RemoveWeight and EditWeight. The model calls directly into the undo
command, which is somewhat unfortunate as it feels like a layering
violation. It's the easy thing to do for now.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
72c6b83866