Added listbox::clear_headers() and menubar::clear()

Added listbox::clear_headers() to remove all the columns (included categories and items) from the listbox
Added menubar::clear() to remove all the items from menubar

source/gui/widgets/listbox.cpp

@@ -5328,6 +5328,7 @@ namespace nana
 		{
 			internal_scope_guard lock;
 			auto & ess = _m_ess();
+			ess.lister.erase();
 			ess.header.clear();
 			ess.update();
 		}