`ChecklistEditor` does not update checkboxes to reflect internal state when items change

While the `CheckListEditor` correctly updates the checkboxes if the list is set to a new value (via the usual `update_object` path), there is nothing listeneing to the `items` of the list, and so in-place changes in the model don't update the UI.

This can be worked around by explicitly setting the trait, but it would be better to have listeners to the `items` of the value, if possible.