Add QQmlApplicationEngine::clear() to delete root objects after clearComponentCache() is called

void QQmlApplicationEnginePrivate::finishLoad(QQmlComponent *c)
{
    Q_Q(QQmlApplicationEngine);
    switch (c->status()) {
    case QQmlComponent::Error:
        qWarning() << "QQmlApplicationEngine failed to load component";
        qWarning() << qPrintable(c->errorString());
        q->objectCreated(0, c->url());
        break;
    case QQmlComponent::Ready: {
        auto newObj = c->create();
        objects << newObj;
        QObject::connect(newObj, &QObject::destroyed, q, [&](QObject *obj) { objects.removeAll(obj); });
        q->objectCreated(objects.constLast(), c->url());
        }
        break;
    case QQmlComponent::Loading:
    case QQmlComponent::Null:
        return; //These cases just wait for the next status update
    }