void MainWindow::on_buttonAdd_clicked()  { // when "Add" is clicked
    QStringList filenames =  QFileDialog::getOpenFileNames (this, "Choose files");
    if (filenames.empty ())
        return;

    for (auto i = 0; i < filenames.size (); ++i) {
        ui->listPlaylist->addItem (filenames[i]);
    }
}

void MainWindow::on_listPlaylist_itemDoubleClicked(QListWidgetItem *item) { // when double-clicked item on QWidgetList
    set_track (item->text ());
}