a11y: QMenu popup not in a11y registry

When using a popup QMenu it doesn't get exposed in the accessible tree registry of at-spi. Unclear why.

• use the tool `accerciser` to inspect Accessible entities of an application (it's a bit tricky to ensure the menu stays open while fiddling with accerciser, I've started the app on a nested wayland compositor so the menu doesn't lose focus when switching to accerciser)
• no trace of QMenu in accerciser, *even when it is currently popped up*
• it strangely appears to send `showing` state changes when monitoring events of the application, so I presume the Accessible is actually exported on the a11y bus, it just does not appear in the tree

// SPDX-License-Identifier: BSD-2-Clause
// SPDX-FileCopyrightText: 2023 Harald Sitter <sitter@kde.org>

#include <QApplication>
#include <QMenu>
#include <QPushButton>

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    auto w = new QWidget;
    auto b = new QPushButton(w);

    auto l = new QMenu(w);
    l->addAction(new QAction("hello"));
    QObject::connect(b, &QPushButton::clicked, l, [l] { l->show(); });

    w->show();

    return app.exec();
}