Followup to commit 26aeb516 - further UML2 alignment for loading Umbrello XMI into PapyrusUML

umbrello/umlmodel/folder.cpp
- At function save1 :
  - Update documentation.
  - In UML2 mode, when saving a predefined view create a
    <packagedElement xmi:type="uml:Model"> element.
- At function saveToXMI improve documentation.
- At function load1 :
  - Update documentation.
  - On comparing `type' with "packagedElement" and "ownedElement" a
    simple equality test suffices.

umbrello/umldoc.cpp
- In function loadUMLObjectsFromXMI for-loop of node test `type'
  against "packagedElement" alongside testing
  tagEq(type, QLatin1String("Model")). Reason: The predefined folders
  are now generated as <packagedElement>s.

CCBUG: 115269