EXT-7104 WIP Added "Sidebar" button (disabled until callback is set)

* Added widgets into xml, added setting and listener for context menu
* added menu item to show/hide the button
* button is made as may be hidden on resize

Additional improvements:
* Updated method to process buttons width when it is necessary to show any button via context menu
* Updated LLBottomTray::initButtonsVisibility to not show warning messages if it is impossible to show a button due to bottom tray width on startup.

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/403/

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branch : product-engine