[REG 5.14-5.15] Window(QGLWidget in QGraphicsView) reparenting regression in 5.15

Ref https://bugs.kde.org/show_bug.cgi?id=427569

The code creates a new `QWidget` without a parent, set it to visible before adding it to a container. It used to work on 5.14 and earlier (and has been there for at least 8 years). However, on 5.15, the set visible creates a toplevel window that remains toplevel even after the widget obtained a parent later.