umbrello/dialogs/pages/classgeneralpage.cpp crash fix for UMLInstance

- In ClassGeneralPage(UMLDoc*, QWidget*, UMLObject*),
  if `t' is UMLObject::ot_Instance then
  - add local UMLInstance *inst as shorthand for
    m_pObject->asUMLInstance();
  - rename existing local className to classNameLabel;
  - add local className of type QString;
  - if inst->classifier() is non null then set className to
    inst->classifier()->name();
  - on constructing new UMLObjectNameWidget provide arguments
    classNameLabel, className.