BHV-6685: Adjust z-index of DataGridList item when spotted/unspotted.

Issue
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When a DataGridList item contains a Marquee, there is a significant delay in starting and stopping the Marquee, and more generally the transition. This caused delays in responding to user input and scrolling through lists.

Fix
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Upon spotting of a DataGridList item, we set the `z-index` property to a value one larger than the current value. Rather than calculating the `z-index` value of every item, we make the assumption that all items in the DataGridList will have the same `z-index` value; alternatively we can keep track of the highest `z-index` value when the DataGridList is rendered and set the `z-index` of the spotted item to one larger than that, but it seems like a very rare case that there would be different `z-index` values for items in a grid and adds unnecessary overhead upon creation. Let me know if we should reconsider this assumption.

When unspotted, we remove our previously applied `z-index` style, using a setTimeOut with 0 delay so that we ensure this executes after the transition has ended, which is critical to maintaining performance.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>