Improve support for combo boxes in all browsers (#8687)
* Improve support for combo boxes in Firefox / Chrome / Internet Explorer.
* UIABrowseModeDocument's __contains__ method: ensure that the given object is a descendant of the document by searching the ancestry for the document's runtimeID with a treeWalker.
* Powerpoint appModule: remove event_gainFocus from NVDAObject class that has a focusRedirect property as this is now handled in eventHandler.
* browseMode's collapseOrExpand script: do not mark as allowed to run when in focus mode.
* Update what's new
