Clears either all the callbacks or the callbacks for a particular
        signal.
        
        :param      signal | <variant> || None

def clear(self, signal=None):
        """
        Clears either all the callbacks or the callbacks for a particular
        signal.
        
        :param      signal | <variant> || None
        """
        if signal is not None:
            self._callbacks.pop(signal, None)
        else:
            self._callbacks.clear()