Subclass of QMessageBox which re-implements the static public
functions. There are two reasons for this:
1) The QMessageBox implementation on OSX does now show the title
string. This leads to message boxes which don't make much sense. So on
OSX we set title to text and text to informative text.
2) If parent is NULL, we set parent to MainWindow::instance. This
prevents message boxes which can go behind MainWindow

Changed NUM_OUTPUT_SIGNALS to an const unsigned short from a #define and moved its definition into OpalRT.h from OpalLink.h so that the configuration would all be in one place.