add a Sirius service that returns true is the model element has changed (ie. new value)

Currently, we have 2 services: (related to domain specific action)
* hasBeenActivated : ie. the last object(s) that have run (for example the "self/target" model element on which a rule or action has been called)
* isCurrentInstruction: ie. the "self/target" model element of the action of the selected step element in the stack/debug view

As suggested by @ebousse it would be interesting to have a service that is driven by actual value changes. This can be something similar to the yellow highlighting in the Variable view.
This may be related to the trace. so we can hightlight all element that have changed in the given step (big or small step)
it may also depend on the selection in the debug/stack view or the selection in the timeline ?