QQmlListProperty: prepare for transition to QList-free implementation

QQmlListProperty(QObject *object, QList<T *> &list) exists; a similar constructor taking a QVector does not. So if your storage is QVector you have to do more work to expose it as a QQmlListProperty (it can be done). The difficulty should be the other way around: easy to use QVector, but deprecate usage of QList ASAP.

The name is unfortunate, but it's a public class. If we consider "list" to mean "sequence of objects" maybe it's OK.

If we want to stop using QList in QtQuick APIs in Qt 6, we'd better get started with the transition now - at least make sure that new QList-based API doesn't get added to new classes.