Made it possible to add items to parents of reference items. Four deep.
So a MPartstack can be added to the parent of the parent of a view and
sit nicely beside that stack.

Example:

<extension
  point="com.remainsoftware.e4.model.importer.modelimport">
   <model
  modelURI="platform:/plugin/com.remainsoftware.e4app/Application.e4xmi"
  modelId="partstack"
  referenceId="com.remainsoftware.e3app.view"
  relationship="third">
    </model>
</extension>