'Register a text key.
If the key you need to register is a simple one-to-one mapping
of MP4 atom name to EasyMP4Tags key, then you can use this
function::
EasyMP4Tags.RegisterTextKey("artist", "©ART")'

@classmethod
def RegisterTextKey(cls, key, atomid):

    def getter(tags, key):
        return tags[atomid]
    def setter(tags, key, value):
        tags[atomid] = value
    def deleter(tags, key):
        del tags[atomid]
    cls.RegisterKey(key, getter, setter, deleter)
