LayoutEngine/ELK layouting does no work well with custom source models

[tortmayr]

see #1168

[planger]

Sketch of a potential solution:

• Run auto-layouting as is. This will update the GModel directly.
• Walk through the GModel elements and create a ChangeBoundsOperation containing the new position of each element with bounds
• Similarly, create ChangeRoutingPointsOperation for routing points of SRoutableElementImpl instances
• Use the action dispatcher to dispatch all created change bounds and change routing point operations

This will allow reusing the diagram-specific implementations of those operations so the position and routing point changes would correctly end up in whatever source model is being used.

The behavior sketched above should probably be done in the LayoutOperationHandler after it has called layout().

[tortmayr]

Side note: some default implementations for handling the ChangeBoundAction might not suited to handle this use case.
e.g. the EMFChangeBoundsOperationHandler currently only handles top-level shapes. So layout information of ports and or sub-labels would get lost.