Sets the default for a given key to the value.
        
        :param      key | <str>
                    value | <str> || <XResourceManager>

def setDefault(self, key, value):
        """
        Sets the default for a given key to the value.
        
        :param      key | <str>
                    value | <str> || <XResourceManager>
        """
        if value is None:
            self._defaults.pop(key, None)
        else:
            self._defaults[key] = value