The lists defining a menubar or toolbar are very similar.	Each is a list with named components. A component is a
	terminal node if it has a handler component, which is a
	function to be called (without arguments) when the menu item
	or toolbar item is selected. Optionally, an icon
	component can be given specifying a stock icon to accompany
	the text. A non-null component named separator will
	also indicate a terminal node. In this case, a visible
	separator will be displayed.
	A menubar list can use the hierarchical nature of a list to
	generate submenus. For toolbars this is not the case.
	These constructors map the list into the widget. The methods
	for the constructors refer to these list defining the widget.
	The svalue method returns the list.
	The svalue<- method can be used to change the list, and
	hence redo the menubar.
	The "[" method refers to the components of the list.
	The code{"[<-"} method can be used to change pieces of the
	menubar or toolbar.
	The add method with signature (obj,lst) or
	(obj,gmenu.instance) can be used to apped to the
	current menubar/toolbar. The second argument is a list or an
	gmenu or gtoolbar instance.
	The delete method 
	can be used to delete part of the menubar/toolbar. The
	value argument can be either a character vector with
	the top-level names to delete, or a named list, or an instance
	of either gmenu or gtoolbar.
	Popular usage reserves toolbars and menubars for top-level
	windows -- not sub windows -- as such, the container, specified
	at construction, should be a top-level gwindow instance