Fixed normal EXT-6997 (Fav bar: Pop-up menu for LM in overflow dropdown is...

Fixed normal EXT-6997 (Fav bar: Pop-up menu for LM in overflow dropdown is kept after LM was dragged away)

- Added menu hiding on left mouse down callback for drop down menu item

Reviewed by Mike Antipov at https://codereview.productengine.com/secondlife/r/337/

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branch : product-engine