Initial work on loading UML2/XMI2 files, e.g.

  https://bugs.kde.org/attachment.cgi?id=86180 (Eclipse UML2)
and
  http://www.omg.org/spec/UML/20110701/Infrastructure.xmi

umbrello/model_utils.{h,cpp}
- New function getXmiId(QDomElement) encapsulates access to xmi.id (XMI-1)
  and xmi:id (XMI-2)

umbrello/{umlobject,folder,operation}.cpp
- Replace direct accesses to QDomElement::attribute("xmi.id") by
  Model_Utils::getXmiId()

umbrello/{folder,package}.cpp
- In function load(QDomElement&), decode UML2 tags packagedElement and
  ownedElement.

umbrello/umldoc.cpp
- In function loadFromXMI(QIODevice&, short), decode outerTag values
  "Model" and "Package" for UML2;
- In function loadUMLObjectsFromXMI(QDomElement&),
  - decode UML2 tags packagedElement and ownedElement;
  - admit XMI2 syntax "xmi:id" to the check for xmi.id presence.

umbrello/classifier.cpp
- In function makeChildObject(const QString&), support UML2 tags
  ownedOperation and ownedAttribute.

CCBUG:56184