OTP Token add failed because of invalid function call. qr_widget doesn't
contain `on_value_changed` method since it inherits from `IPA.widget` and
not from `IPA.input_widget`.
Emitting the event was preserved for future possible usage.
https://fedorahosted.org/freeipa/ticket/4306
Reviewed-By: Adam Misnyovszki <amisnyov@redhat.com>