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