Provide More Functionality to QQmlListProperty

Please expand functionality of QQmlListProperty with methods such as insert, remove, take, etc. Also, for that matter, please provide consistent accessor functions for all model types.

For example, after a list<QtObject>, a QList<QObject*> exposed from C++, a ListModel, etc. has been assigned to a QQmlListProperty, accessing the model data via the QQmlListProperty should be consistent (count, length, or size should all be available, so that internal implementation of classes using a QQmlListProperty do not require one type of list or another to work). Indexing techniques should all also be available (.get, .at, [i]) for the same reason.