Change metamodel to reference UML elements

- Change domainElements in DomainModel to containment so that we can add domain elements in the tree editor to the domain model and not as
isolated artifacts. 
- Import UML2 metamodel into the IFML metamodel and all the required
dependencies. 
- Change UMLStructuralFeature, UMLDomainConcept, UMLBehavioralFeature
and UMLBehavior classes to reference the right classes from UML2
metamodel.