Decide whether we shall use qCritcal, qWarning, qInfo

It looks like we are rather undecided whether or not we should use the error handling functions from QtGlobal. Right now, we have

• 2 calls of qCritical,
• 1 call each of qWarning and qInfo,
• 0 calls of qFatal.

In GUI/main/MessageHandler.cpp, a call to QMessageBox::warning is outcommented. Messages are printed to std::cerr; hence they are invisible for Windows users.

How does all this relate to our new logger?