Add support for the rubber bands to be top-level

Worksaround a bug where they can't be transparent when embedded
in a MFC application

src/private/indicators/ClassicIndicators_p.h

@@ -49,8 +49,10 @@ Q_SIGNALS:
 private:
     friend class KDDockWidgets::Indicator;
     friend class KDDockWidgets::IndicatorWindow;
+    bool rubberBandIsTopLevel() const;
     void updateIndicatorsVisibility(bool visible);
     void raiseIndicators();
+    QRect geometryForRubberband(QRect localRect) const;
     void setDropLocation(DropLocation);
     void updateWindowPosition();