Implement accessibility enhancements (issue #1217)

- Add new CefBrowserHost::SetAccessibilityState method for toggling
  accessibility state when readers are detected by the client.
- Add new CefAccessibilityHandler interface for the delivery of
  accessibility notifications to windowless (OSR) clients.
- Fix delivery of CefFocusHandler callbacks to windowless clients.
- cefclient: Add example windowless accessibility implementation on Windows and macOS.
- cefclient: Automatically detect screen readers on Windows and macOS.

tests/cefclient/browser/browser_window_osr_gtk.h

@@ -80,6 +80,7 @@ class BrowserWindowOsrGtk : public BrowserWindow,
       CefRefPtr<CefBrowser> browser,
       const CefRange& selection_range,
       const CefRenderHandler::RectList& character_bounds) OVERRIDE;
+  void UpdateAccessibilityTree(CefRefPtr<CefValue> value) OVERRIDE;
 
  private:
   ~BrowserWindowOsrGtk();