Add methods to compute the nearest position on a scrollList item

## Problem
Provide method(s) to compute a location on a ScrollList item that is nearest to an event position.

## Solution
Add `ScrollList#restrictPositionToItemContainer` and `ScrollList#restrictPositionToNearestItem`.

## How to QA/+10
1. CI Build Passes
1. Check out the `add_getPositionOnNearestPage_to_scrollList` branch
1. Run `./init.sh && grunt qa`
Does this need to be public? The less we have in the API the better.