This attribute stores the address of the gadget's target object.  Whenever
the gadget receives an om_notify message, it sends an om_update message to
its target.  If the gadget has an attribute mapping list (see the ica_map
attribute below), it also maps the IDs from the OM_NOTIFY message.

If the value of ICA_TARGET is ICTARGET_IDCMP, the gadget sends an
idcmp_idcmpupdate intuimessage to its window.  see the rootclass
description of om_update for more information.