<oTopBar>:setItem(<nPosition>, <oMenuItem>) --> self

        <nPosition> is a numeric value that indicates the position in the top
        bar menu of the item that is being retrieved.

        <oMenuItem> is the MenuItem object that replaces the one in the top
        bar menu specified by <nPosition>.

        setItem() is a method of the TopBarMenu class that is used for
        replacing a MenuItem object after it has been added to a top bar
        menu.  After the setItem() method is called, the display() method
        needs to be called in order to refresh the menu.