Followup to commit 3203b16a - Clean up ClassifierWidget construction:

- The constructor which had originally taken a UMLClassifier* argument
  had been changed to take UMLObject* in order to be usable also for
  UMLInstance.  Changing the argument type to UMLObject* introduced an
  error in runtime method selection due to overlap with the constructor
  taking UMLPackage* argument.  Solution:

umbrello/umlwidgets/classifierwidget.{h,cpp}
- At constructor taking UMLObject* argument revert argument type to
  UMLClassifier*.
- Add new constructor taking UMLInstance* argument.

CCBUG: 123633