Adwaita: Don't do n-th child things for list rows

Adwaita: Don't do n-th child things for list rows

Making the list row child css depend on the position
is very expensive, and does not acutally work correctly
(since we don't have widgets for all children, so the
position of the child widget does not reflect the actual
model item position).

To make this more palatable, use the bottom border
instead of the top border, since most lists have a natural
border at the top (with headers), and may end up with
empty space at the bottom.