views: Support top menu switching with mouse and arrow keys (issue #2102)

- cefclient: Add example ViewsMenuBar implementation and show top menu
  by default (specify `--hide-top-menu` to hide it).

libcef/browser/views/menu_button_impl.h

@@ -32,6 +32,7 @@ class CefMenuButtonImpl :
   void ShowMenu(CefRefPtr<CefMenuModel> menu_model,
                 const CefPoint& screen_point,
                 cef_menu_anchor_position_t anchor_position) override;
+  void TriggerMenu() override;
 
   // CefLabelButton methods:
   CefRefPtr<CefMenuButton> AsMenuButton() override { return this; }