Add the Qt::ItemNeverHasChildren flag and use it in QTreeView.

It can be used to determine whether expand() should really expand.

Change-Id: If79d8c295a4ca1356e60051682b227524a065126
Reviewed-by: David Faure (KDE) <faure@kde.org>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>