Add QSerialPort::waitFor*(std::chrono::milliseconds)

In QtCore, to express timeouts we try to migrate from int-based APIs to std::chrono-based (see e.g. QTimer).

Let's try to do the same for QSerialPort::waitForBytesWritten()/QSerialPort::waitForReadyRead(). Of course, we need to keep the old functions for BC/SC reasons, so this task is about adding new overloads.