Sets the group leader window for this.
By default, GDK sets the group leader for all toplevel windows to a global window implicitly created by GDK. With this function you can override this default.
The group leader window allows the window manager to distinguish all windows that belong to a single application. It may for example allow users to minimize/unminimize all windows belonging to an application at once. You should only set a non-default group window if your application pretends to be multiple applications.
a toplevel Window
group leader window, or null to restore the default group leader window