EXT-7104 WIP added 4 buttons to bottom tray, added "Show button" menu support.

* Added 4 buttons with tooltips:
 ** Build - Shows/hides Build Tools
 ** Search - Shows/hides Search
 ** Map - Shows/hides World Map
 ** Mini-Map - Shows/hides Mini-Map
* Made their width and minimal width the same as "Move" and "View" buttons (increased default total width of the bottom tray to make Layout Stack initialize its panels with default values from xml)
* Added appropriate menu items in context menu to Show these buttons in the bottom tray (with functionality)
* Set on click actions for this buttons. ("Build" temporary made disabled because default toggle registration is not enough for this floater.)

Dev Notes:
* Improved LLBottomTray::initResizeStateContainers to fill mObjectDefaultWidthMap using mStateProcessedObjectMap & mButtonsProcessOrder container
* Improved LLBottomTray::canButtonBeShown to process variable number of buttons that can be hidden on resize

QA Notes:
Resize behavior with new buttons can already be tested.

Reviewed by Vadim Savchuk at https://codereview.productengine.com/secondlife/r/381/

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branch : product-engine