Support in-cell editing in Excel 2016/365 (#9290)
* UIAHandler: remember the last UIA element to receive a UIA focus event.
* Support in-cell editing in Excel 2016/365
* UIA: catch a COMError when trying to make a cache request.
* Excel: address review comments: cache focusRedirect, comment why parent must be set. also increase search count, and only set parent if previous focus was an ExcelCell.
* Update what's new.
