[Proposal] Make Changed node be able to detect changes of mutating objects via ticketing

[gregsn]

check whether IMutatingObject.Ticket has changed.

Discuss: Is it possible to automatically implement ticketing for classes where the user implements IReadOnlyTreeNode and enables the feature (checkbox in the class config)? The system would create new tickets whenever a property changed or a Children ticket changed. It might still be necessary to trigger the computation (at least in order to detect indirect hierarchical changes).