Marks window as attached to attach_widget. This creates a logical binding between the window and the widget it belongs to, which is used by GTK+ to propagate information such as styling or accessibility to window as if it was a children of attach_widget.
Examples of places where specifying this relation is useful are for instance a gtk.Menu created by a gtk.ComboBox, a completion popup window created by gtk.Entry or a typeahead search entry created by gtk.TreeView
Note that this function should not be confused with Window.setTransientFor, which specifies a window manager relation between two toplevels instead.
Passing NULL for attach_widget detaches the window.