QWidgetWindow::event(): Call base implementation for QEvent::WindowStateChange

The base implementation takes care of updating visibility and emitting
signals.

Task-number: QTBUG-59313
Change-Id: I270b37c894420902488d89dc0c79f4c12b8d9a29
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/widgets/kernel/qwidgetwindow.cpp

@@ -285,6 +285,7 @@ bool QWidgetWindow::event(QEvent *event)
         return true;
 
     case QEvent::WindowStateChange:
+        QWindow::event(event); // Update QWindow::Visibility and emit signals.
         handleWindowStateChangedEvent(static_cast<QWindowStateChangeEvent *>(event));
         return true;