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:

-- |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.
Merge pull request #23 from rootzlevel/list-item-add-event

Report correct item position in NewItemEvents