Add indirection in View API to allow view type to transform view changes

[JanWittler]

This PR provides two contributions:

• Instead of allowing a view to directly commit to the underlying model, an indirection over the view type is introduced. When creating a view with a non-identity mapping, this function encapsulates the necessary backwards transformation from changes corresponding to instances of the view metamodel to changes corresponding to instances of the underlying model metamodel.
  • As a side effect, the propagated changes are not exposed anymore to the view.
  • Currently, we only have the IdentityMappingViewType which does not need to transform changes. However, with the migration to UUIDs this API refinement also allows to use hierarchical IDs in the view and assign UUIDs in the view type, thus completely hiding the UUIDs from the view.
• Refines the StateBasedChangeResolutionStrategy to return unresolved changes. This was changed as previously the elements of the internal intermediate ResourceSet were exposed.

TODOs

• create tests for IdentityMappingViewType:commitViewChanges