Allow editors.List subclass to define their ListItem class

Hello,

I'm writing a new editor based on List and I need it to use another List.Item class.
I didn't want to override the whole List.Item as I want to be able to use both editors (my extension and the original List).
Here's how I did it. Tests passed successfully on firefox and chromium (and I don't see why it wouldn't).

I wonder if it can be useful to allow to override this in the schema. Let me know what you think.

Have a nice week!