(qmltc) Use QQmlListProperty<QObject> instead of QQmlListReference in generated code

Whenever one creates a list<X> property in QML and then assigns something to it via a binding (relevant for QObject * list properties and their assignments), qmltc generates code that creates QQmlListReference and then uses its interface to append elements to the list property.

This is rather slow* and in reality we could directly use QQmlListProperty<X> API instead (the assumption is that any "list" property in QML is a QQmlListProperty**).

Rough code generation:

import QtQuick
Item { // item has a default property "data" which is a QQmlListProperty
  Text {} // default property assignment here would append to "data"
}

// rough code generated to assign a Text object:

// old:
QQmlListReference data(item, "data"); // item is a QQuickItem *
data.append(text); // text is a QQuickText *

// new:
QQmlListProperty<QObject> data = item->data(); // call QQuickItem::data() directly
data.append(&data, text);

* QQmlListReference looks up a property in a QObject by name and then does extra boilerplate that we don't need in qmltc.
** except list of value types in which case assume it is a QList but this is out of scope here