gio:menu-prepend-submenu (menu label submenu)

menu -- a g:menu object
label -- a string with the section label, or nil
submenu -- a g:menu-model object with the items of the submenu

Convenience function for prepending a submenu to the start of the menu. Combine the g:menu-item-new-submenu and g:menu-insert-item functions for a more flexible alternative.