[QT] convert a QString to char*

See the following example for a demonstration:

先將QString型態的字串，透過QByteArray將字串轉換成字元陣列。

int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    QString str1 = "Test";
    QByteArray ba = str1.toLocal8Bit();
    const char *c_str2 = ba.data();
    printf("str2: %s", c_str2);
    return app.exec();
}