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_dll/ctocpp/menu_model_ctocpp.h

@@ -31,6 +31,7 @@ class CefMenuModelCToCpp
   CefMenuModelCToCpp();
 
   // CefMenuModel methods.
+  bool IsSubMenu() OVERRIDE;
   bool Clear() OVERRIDE;
   int GetCount() OVERRIDE;
   bool AddSeparator() OVERRIDE;