QMainWindow: Clear menubar parent when new one is set

In QMainWindow::setMenuBar(), we hide and schedule the current
menubar, if any, to be deleted later. However, it remains installed
as its whole ancestry's event filter, which could conflict with the
newly assigned menubar until the old menubar is destroyed. In our
case, we have noticed issues with the Cocoa QPA plugin.

We force uninstalling the old menubar as event filter by setting its
parent to null, pending its deletion shortly after.

This fixes BigMenuCreator's empty menubar when calling it with only
the "--new-menubar" option. It also fixes QTBUG-34160 example which
was not behaving as well as it should.

Task-number: QTBUG-34160
Change-Id: Ifefb72affad01e7b7371005442074afd6a39a5b8
Reviewed-by: Dmitry Shachnev <mitya57@gmail.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/widgets/widgets/qmainwindow.cpp

@@ -574,6 +574,7 @@ void QMainWindow::setMenuBar(QMenuBar *menuBar)
                 menuBar->setCornerWidget(cornerWidget, Qt::TopRightCorner);
         }
         oldMenuBar->hide();
+        oldMenuBar->setParent(nullptr);
         oldMenuBar->deleteLater();
     }
     topLayout->setMenuBar(menuBar);