I can undo a single change to a Thing

(User Story, To be Fixed , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
History
Summary: From the History View, I should be able to look at a specific Property change, say "undo", and change it back to the value before that change.
(This is a significantly rewritten version of this Issue.)
We should probably show a confirmation box with the value to be reverted to, before making the change.