EXT-8357 FIXED ESC key sometimes failing to de-focus the sidebar.

I've found out that the only case when pressing ESC doesn't transfer focus from side tray to game area is
when a flat list view (people, groups, teleport history, etc) is focused.
So, I fixed LLFlatListView to handle pressing ESC as expected.

Reviewed by Sergey Litovchuk at https://codereview.productengine.com/secondlife/r/862/

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branch : product-engine