This was the missing counter-part to indexOf(QWidget *), which is
sometimes implemented in user code.
Not sure why the original code doesn't use a for-loop and instead accesses
an out-of-bounds element, but I'll preserve the behavior of very old working code.
Change-Id: I7d7fa56b56a4626789774c15c23fdfef41d723e7
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>
7a342372bb