'Add a translatable string to this widget
@param key: the key for the string
@param string: the string, or None to remove the key'

def setstr(self, key, string=None):

    ref = ('%s:%s' % (self.ref, key))
    strings = self._strings
    if string:
        if hasattr(string, 'flatten'):
            string = string.flatten()
        strings[ref] = string
    elif (key in strings):
        del strings[ref]
    return
