The contents of a QQmlListProperty are mutable even if the property is
not. This is because QQmlListProperty is only a view on a different
container.
Pick-to: 6.7
Fixes: QTBUG-123196
Change-Id: Id6309b1e1ddc219bf35e8d9888b8415dcc0f9d43
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Ulf Hermann