Undo and redo API

[cdamus]

• undo and redo commands on the edit endpoint
• on undo, compute the inverse of the command that was undone and send that to subscribers for incremental update notification
• on redo, send the redone command to subscribers
• ensure that the save API tells the CommandStack that save was done
• if undo returns to the last savepoint, notify subscribers about dirty state change (command-stack listener)