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).

include/cef_menu_model.h

@@ -59,6 +59,12 @@ class CefMenuModel : public virtual CefBaseRefCounted {
   static CefRefPtr<CefMenuModel> CreateMenuModel(
       CefRefPtr<CefMenuModelDelegate> delegate);
 
+  ///
+  // Returns true if this menu is a submenu.
+  ///
+  /*--cef()--*/
+  virtual bool IsSubMenu() =0;
+
   ///
   // Clears the menu. Returns true on success.
   ///