This method tells a gadget to send an om_update message to its target
object.  Boopsi gadgets have a function similar to icclass objects--each
gadget can have an ica_target and ica_map in order to notify some target
object of attribute changes.  When a Boopsi gadget sends an OM_NOTIFY
message, it always includes its GA_ID.  This makes it easy for an
application to tell which gadget initially sent the OM_NOTIFY.  See the
description of icclass's om_notify and om_update and the rootclass's
om_notify and om_update methods for more details.