BHV-743: Adjust scroller edge when navigating to first or last row.

Issue
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The "up" paging control is not deactivated when the top of the list is reached via 5-way (and likewise, the "down" paging control is not deactivated when the bottom of the list is reached).

Fix
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We pre-compute the index bounds for the first and last rows, and compare the currently spotted/focused item's index with these bounds, scrolling to the top or bottom accordingly. Additionally, I consciously chose to break out the column value into a separate variable `columnSize` for the sake of code readability, as we could have just used `this._indexBoundFirstRow` instead.

Notes
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Open question for review - will there be any side effects, UX or otherwise, from programmatically scrolling to the top/bottom in these cases (versus trying to adjust the bounds which the paging controls consider to be the "top" and "bottom")?

This PR is dependent on an Enyo PR: https://github.com/enyojs/enyo/pull/729

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>