a11y: App with simple QWidget as toplevel lacks toplevel in AT-SPI

Qt doesn't require having a QDialog/QMainWindow/... as a top-level in the widget hierarchy, but allows having any QWidget as the top-level.
Using a simple QWidget as the top-level however currently comes with the problem that the window isn't reflected in the accessibility hierarchy, as only widgets are reflected in the accessibility hierarchy, while QWindow is not.

As a consequence, Qt apps using a simple QWidget as the top-level don't report any object with top-level role on the AT-SPI level, which is incorrect.

Steps to reproduce:

1. Build and run a simple sample program like the following:

#include <QApplication>
#include <QLabel>
int main(int argc, char* argv[])
{
    QApplication a(argc, argv);
    QLabel label("My label");
    label.show();
    return a.exec();
}

1. start Accerciser and take a look at the accessible hierarchy of the sample application

Actual result
The app only has a single top-level child with role "label"

Expected result:
The app should have a top-level object with the AT-SPI "frame" role to represent the window. That top-level object should have the object with role label as a child.

Further information:

• The Orca screen reader currently needs a workaround because of this: https://gitlab.gnome.org/GNOME/orca/-/commit/d0a0b84c7cbb0aa560a58527e652823a90a77992 (Quoting from the commit message: "Unfortunately, some Qt apps don't expose valid roles
  for the top-level object. The top-level object should never have a
  role of filler, for instance.")
• This was discussed in https://codereview.qt-project.org/c/qt/qtbase/+/592200/comments/20d2250f_efecdd07 earlier, with the suggestion to create an issue to keep track.
• The behavior is as expected when using a "proper" top-level widget as the top-level in the Qt application, e.g. a QMainWindow or a QDialog, for example:

#include <QApplication>
#include <QLabel>
#include <QMainWindow>
int main(int argc, char* argv[])
{
    QApplication a(argc, argv);
    QMainWindow window;
    QLabel* label = new QLabel("My label");
    window.setCentralWidget(label);
    window.show();
    return a.exec();
}

However, if Qt doesn't require app developers to have a "proper" top-level in the widget hierarchy, it should in my opinion still take care of exposing one on the accessibility level (e.g. for the associated QWindow).