public interface TopLevelMenuContainer
A markup interface to indicate this is a top level menu or command bar. The original Swing code used JMenuBar to
determine if it is TopLeveMenu. However since we introduced CommandBar, this criteria is not correct anymore. The new
condition is if a container implements TopLevelMenuContainer, the children in that container is top level menu. If
isMenuBar returns true, it means the container is really a menu bar, just like JMenuBar.