`Grid` scrolling timeout for pointer events can be customized now via a prop; same default value is preserved as before (150ms).

Also addressed a couple of small bugs as well:
* Multiple `WindowScroller` instances can be used on a single page now without interfering with each other's `pointer-events` settings.
* Calling `recomputeGridSize` on `Grid` (or any of its wrapping HOCs) will clear any pending cell cache to avoid edge-case issue where a scrolling `Grid` has invalid cached cells due to a change in the underlying collection data.
