Fix feadback changes in TupleEditor

This PR fixes the feadback changes caused by updating programmatically the trait tuple value in the `model`


So this function is called 3 times in your test, but only the first time the value is updated, is that right? Wouldn't it be cleaner to have two notifications, one with `@on_trait_change('fields[]')`, and one with `@on_trait_change('fields')`?