Starts the internal timer.
        
        :param      interval | <int>

def _startTimer(self, interval):
        """
        Starts the internal timer.
        
        :param      interval | <int>
        """
        if not self.__timer:
            self.__timer = QtCore.QTimer(self)
            self.__timer.setSingleShot(self.__singleShot)
            self.__timer.setInterval(interval)
            self.__timer.timeout.connect(self.timeout)
            self.__timer.destroyed.connect(self._clearTimer)

            # ensure to stop this timer when the app quits
            QtCore.QCoreApplication.instance().aboutToQuit.connect(self.__timer.stop, QtCore.Qt.QueuedConnection)
        
        self.__timer.start(interval)