Refactor UMLInstance to be more UML and XMI 1.4 compatible.

- Create classifier instance on construction if not present
  and save it internal instead to be a non sharable classifier.
  Class UMLInstance itself stays to be derived from UMLClassifier
  because of the usage of UMLAttribute representing instance
  attributes.
- Save and load 'classifier' attribute to be xmi 1.4 compatible

CCBUG:123633