Replace usage of UMLObject::m_pUMLPackage by QObject member 'parent' through new method umlParent().

In the past UMLObject::m_pUMLPackage has been used in some classes,
while other uses the QObject class member 'parent'. This commit unify
both approaches by using QObject class member 'parent' in all classes.

class UMLObject now has two new functions setUMLParent() and umlParent()
to provides a UMLObject based interface to the parent uml object;
method umlPackage() is now a shortcut of umlParent()->asUMLPackage().

CCBUG:368282
Reviewed-by: aaron.nottbeck at sag.eu