Fix shortcuts in dock widgets with custom title bar

QWidgetResizeHandler cannot be sure that it will see the MouseReleaseEvent and therefore buttonDown
could be set wrong (and is set wrong in QGraphicsViews). When the mouse is up, the widget should
not think it is moving or re-sizing the dock widget. A similar fix exists in the code a few lines
above.

Task-number: QTBUG-70596
Change-Id: I52ce487836bc71da8fd7d71f8a89e21b51406d00
Reviewed-by: Sérgio Martins <sergio.martins@kdab.com>
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/widgets/qwidgetresizehandler.cpp

@@ -192,6 +192,7 @@ bool QWidgetResizeHandler::eventFilter(QObject *o, QEvent *ee)
         keyPressEvent(static_cast<QKeyEvent *>(ee));
         break;
     case QEvent::ShortcutOverride:
+        buttonDown &= ((QGuiApplication::mouseButtons() & Qt::LeftButton) != Qt::NoButton);
         if (buttonDown) {
             ee->accept();
             return true;