Report correct item position in NewItemEvents

Adding an item to a list previously emitted a NewItemEvent with the new
size of the list as first parameter even though the documentation says:

```haskell
-- |A new item was added to the list at _the specified position_ with
-- the specified value and widget.
data NewItemEvent a b = NewItemEvent Int a (Widget b)
```

So we now report the position of the new item instead of the list size.