QWidgetWindow: call base class close event impl

Call the QWindow close event handler when processing
close events in QWidgetWindow.

Change-Id: I2b8691735962f6a222a30a847bb18cc6c86b55d4
Task-number: QTBUG-43344
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>

src/widgets/kernel/qwidgetwindow.cpp

@@ -708,6 +708,7 @@ void QWidgetWindow::handleCloseEvent(QCloseEvent *event)
 {
     bool is_closing = m_widget->d_func()->close_helper(QWidgetPrivate::CloseWithSpontaneousEvent);
     event->setAccepted(is_closing);
+    QWindow::event(event); // Call QWindow QCloseEvent handler.
 }
 
 #ifndef QT_NO_WHEELEVENT