EXP-1286 : First pass at Drag and Drop of tools. Not functional. Most hooks into the LLToolDragAndDrop system in to support the new AT_WIDGET and SOURCE_VIEWER

EXP-1264 Chat log shows as blank if closing viewer with chat log open and then opening chat floater on next login
EXP-1271 Remove UI hints

 * Hooked up commands to their respective floaters

  on the localized button labels.

EXP-1211, EXP-1257 : Save and load the button type along with the toolbars, add a force default load option, enforce consistency between menus and toolbars

Toolbar buttons now affect their floaters as indicated in the FUI_Button_states
wiki page -- https://wiki.lindenlab.com/wiki/FUI_Button_states

- Removed all sidetray dependencies and the sidetray itself.

- Also removed LLFloaterSidetrayTab and LLSidetrayListener as unused.

EXP-1205 PROGRESS -- As a User, I want a toybox which will contain all buttons that I can d&d into the toolbars
EXP-1233 FIX -- Populate the toybox floater window with all FUI toolbar buttons indicated as such in the "commands.xml" definition.
EXP-1267 FIX -- Enable/disable buttons in the toybox

* Hooked up button callbacks to the toolbar buttons
* Fixed toybox button enable/disable to function properly and live update as
  buttons change states.
* Removed the toybox toolbar background image

Reviewed by Leyla

EXP-1257 : Implemented loading of toolbar settings per user account. Also factorize a bit and clean up the related saveToolbars code.

EXP-1262 Hitting single letters reserved for shortcuts inworld opens chat but not associated shortcuts (like Mouselook and Fly)
EXP-1266 Communicate > Nearby Chat menu item does not bring up chat floater

fixed button layout for icon only buttons

fixed button layout for icon+text
layout stack now uses floating point precision to avoid clamping panels to 0

EXP-1203, EXP-1221, EXP-1222, EXP-1223, EXP-1225, EXP-1226 : pull from sergeylproductengine/viewer-experience-fui/

better button sizing
also disabled context menu for non-toolbar region