PopoverMenu.openSubmenu

Opens a submenu of the popover. The name must be one of the names given to the submenus of popover with #GtkPopoverMenu:submenu, or "main" to switch back to the main menu.

#GtkModelButton will open submenus automatically when the #GtkModelButton:menu-name property is set, so this function is only needed when you are using other kinds of widgets to initiate menu changes.