Fixes ENYO-527

## Issue
Need to provide API to determine which controls are visible for a DataList and DataGridList.

## Fix
Add `getVisibleControlRange()` to enyo.DataList which returns the `start` and `end` index of the visible controls. For a control to be considered visible, a certain percentage (established by the `visibleThreshold` property) must be visible.

Enyo-DCO-1.1-Signed-off-by: Ryan Duffy (ryan.duffy@lge.com)