Smooth out scrollbar movement in lazy layouts

[rock3r]

Scrollbars have a very jerky movement in lazy layouts when items have different sizes along the scroll axis; this is a fundamentally unsolvable problem, because we don't know the sizes of the items outside of composition and can't know them.

As a result, the scrollbar thumb changes size and position as you scroll along. The same behaviour can be seen with Android's RecyclerView, which has the same "issue". However, the RecyclerView does some smoothing, so this is a lot less jarring while scrolling. We should try to do the same thing...