Add new mechanism for QmlContext property setting

- QWrappedObject provide a new virtual method setQmlContextProperties,
  so that DuSE-MT implementation continues metamodel-agnostic
- QML representation of UML class properties ok

Change-Id: I2b380345051a5971a77ce9fed4e9834e2312ca1b
Reviewed-by: Sandro S. Andrade <sandroandrade@kde.org>