QVariant MyModel::headerData(int section, Qt::Orientation orientation, int role) const
{
    if(role == Qt::DisplayRole) {
        if(orientation == Qt::Horizontal) {
            switch (section) {
            case 0:
                return QString("first");
                break;
            case 1:
                return QString("second");
                break;
            case 2:
                return QString("third");
                break;
            default:
                break;
            }
        }
    }
    return QVariant();
}