Stunner - BPMN (backend) serialization reusing ID

In order to make the BPMN file "human readable" is important:

• re-use existing id (avoid to change the id to every save of the file)

Notice this is already fixed in the client side marshallers. This ticket refers to the backend implementation.

Acceptance Criteria

• Open BigProcess.bpmn
• Move any node
• Save and check difference before and after the safe.

Expected Result

Only coordinates of moved node will be changed, all ID should remain the same.