umbrello/umlwidgets/diagramproxywidget.{h,cpp} : Address...

umbrello/umlwidgets/diagramproxywidget.{h,cpp} : Address https://mail.kde.org/pipermail/umbrello-devel/2022-January/023025.html
- Add `const' qualifier on following functions:
  const QRectF& clientRect();
  const QRectF& sceneRect();
  UMLWidget*    getProxiedWidget(const QPointF &p);
  bool          isProxyWidget();
  UMLWidget*    getProxiedWidget(const QPointF &p);
  QPointF       mapFromClient(const QPointF &pos);
  QRectF        mapFromClient(const QRectF &r);
  QPointF       mapToClient(const QPointF &pos);
  void          setupEvent(QGraphicsSceneMouseEvent &e,
                           QGraphicsSceneMouseEvent *event,
                           const QPointF & pos);
  void          setupEvent(QGraphicsSceneContextMenuEvent &e,
                           QGraphicsSceneContextMenuEvent *event,
                           const QPointF & pos).
- At the two setupEvent functions, additionally add `const' qualifier
  on argument `event'.