EXT-7104 WIP Implemented a functionality to control the width of the chat entry field.

* LLResizeBar calls notifyParent on resizing before changing rect of view, bottomtray processed this notification to update other buttons.
* Had to move setup of buttons visibility on startup into LLBottomTray to ensure all buttons have necessary visibility BEFORE restore the width of the chat entry field;
* Updated layout between chatbar and speak button to show resize mouse pointer in the middle between them.

Implemented behavior: visible buttons shrink to their minimal width when the chat entry field gets wider.

Also were refactored:
* moved declaration of settings related to buttons visibility from the settings.xml to LLBottomTray.
* moved setting of control listeners to LLBottomTray.

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

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branch : product-engine