Task 782237:
 Wired handling for WinJS.Application's "backpressed" event into _Overlay family of WinJS.UI controls.
  Added Unit tests
  Refactored some code in the _Overlay light dismiss path to make readability and future modifications easier.
  Fixed a systemic Overlay issue where the element in some instances of _Overlay derivative classes still could not recieve programatic focus in non IE browsers. (div's require tabIndex -1).
  Refactored global _Overlay event handler handlers into a mechanism that allows for easy resets to aid unit test scenarios and provide better interoperability with Application.stop()