TreeView QML Type should provide a mapping between visible row and ModelIndex row

The TreeView QML Type API works on the basis of visible rows, e.g. https://doc.qt.io/qt-6/qml-qtquick-treeview.html#expandRecursively-method "row should be the row in the view (table row), and not a row in the model."

We are working now on a TreeModel QML Type QTBUG-130571 and it will (obviously) have a model row based API or a QModelIndex based API to be exact (similar to QmlTableModel (https://doc.qt.io/qt-6/qml-qt-labs-qmlmodels-tablemodel.html).

So at some point we (and our users) will need (or at least benefit) a mapping from visible row to QModelIndex. The right place would imho be the TreeView in the form of e.g.:

Q_INVOKABLE QModelIndex modelIndex(int row, int column);