Add an overload to the QMessageBox.open() method that accepts a slot

The open() method of QMessageBox should accept a slot. This would avoid having to use the SLOT() function and convert the slot method into a C++ method string. This would also make it more in-line with the Signal.connect(Slot) style of connecting signals and slots. Furthermore, PyQt already uses this method style so adding this would improve the compatibility between the two libraries.