ElementImport creation should use createDomainBasedEdge API instead of create

ElementImport are created with AbstractDiagramService#create, which is not correct since ElementImport is a Relationship (even if it is represented as a Node in the Profile Diagram).

This issue also requires a fix in Papyrus UML Services to make sure the imported element and the importing namespace are set when the element is initialized. See org.eclipse.papyrus-domainservices#11 (closed) for more information.