This attribute is used to insert a new gadget into a list of gadgets
linked by their gadget.nextgadget field.  when the om_new method creates
the new gadget, it inserts the new gadget into the list following the
GA_Previous gadget.  This attribute is a pointer to the gadget  (struct
Gadget *) that the new gadget will follow.  This attribute cannot be used
to link new gadgets into the gadget list of an open window or requester,
use addglist() instead.