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.

include/cef_render_handler.h

@@ -40,6 +40,7 @@
 
 #include <vector>
 
+#include "include/cef_accessibility_handler.h"
 #include "include/cef_base.h"
 #include "include/cef_browser.h"
 #include "include/cef_drag_data.h"
@@ -57,6 +58,15 @@ class CefRenderHandler : public virtual CefBaseRefCounted {
   typedef cef_paint_element_type_t PaintElementType;
   typedef std::vector<CefRect> RectList;
 
+  ///
+  // Return the handler for accessibility notifications. If no handler is
+  // provided the default implementation will be used.
+  ///
+  /*--cef()--*/
+  virtual CefRefPtr<CefAccessibilityHandler> GetAccessibilityHandler() {
+    return NULL;
+  }
+
   ///
   // Called to retrieve the root window rectangle in screen coordinates. Return
   // true if the rectangle was provided.