LPS-43889 - DDL Spreadsheet Cells can be highlighted outside of the portlet container

Hi @marclundgren,

Attached is an update for https://issues.liferay.com/browse/LPS-43889.

I originally put this fix into Alloy, but after not being able to reproduce this bug in the AUI datatable demo I decided to put it in the portlet.  YUI datatable has a scrollable property that I leveraged to make it somewhat simpler.

Please let me know if there are any issues.

Thanks!