Allow AnimatedList as a child

I replaced:

`final BoxScrollView child;`

with

`final AnimatedList child;`

and everything worked like a charm. The problem, of course, is that I will have to remember to do it again when and if the plugin updates.

It will be nice to allow AnimatedList as a child. There is already `child.scrollDirection == Axis.vertical`, so passing any Widget with scrollDirection should work.