Code review comment for lp:~mir-team/mir/multiple-change-callbacks

I'm still not convinced by ObserverID - other elements of the scene are managing observers by smart pointers to the observer object (Surface is close to its final state, Session still needs some work).

In all cases the legacy approach of "something changed but I won't tell you what" notification is getting reworked. So these changes will be swept away by that anyway.