[spike] Find out whether QIODevice::readLineInto() is possible to implement without (public) virtual methods

Let's finally find out whether this can be implemented without (public) virtual functions or not (in which case it has to wait for 7.0).

QTextStream has QString readLine() as well as readLineInto(QString*, qint64 max), QIODevice only has QByteArray readLine() and lacks readLineInto(QByteArray*, qint64 max).

The idea behind the -Into-functions is to preserve capacity in the string/byte-array from one line to the next, a la std::getline, unlike readLine(), which allocates a new buffer for each line.