The virtual viewport machinery now handles all of these use cases.
Previous clients of ScrollableWidgetList can use ScrollableLazyList
instead.

Previously this code was in the animation layer, which didn't make much
sense because scrolling is a widget concern.

We were recomputing which widgets to show only when we were on the other side
of the repaint boundaries. That doesn't work well for pageable lists because we
come to rest exactly on a repaint boundary, which means we don't cull the other
page.

After this patch, we recompute the set of widgets using an edge-trigger when we
hit the boundary. That's better than using a level-trigger so that we don't
continuously recompute the set of widget as we sit at the boundary.

We use the ScrollDirection for more than just scrolling.

Fixes #151

This patch moves PageableList off HomogeneousViewport and onto RenderList and
friends, making it match the new ScrollableList.