This method tells an object to broadcast an attribute change to a set of
target objects using om_update messages.  the return value for this method
is not explicitly defined.

The OM_NOTIFY method uses the same message structure as om_update.

Most dispatchers do not handle the OM_NOTIFY message directly.  Normally
they inherit this method from a superclass, so they pass the OM_NOTIFY
message on to the superclass dispatcher.

Although most dispatchers don't have to process OM_NOTIFY messages, most
do have to send them.  Whenever an object receives an om_set or om_update
about one of its attributes, it may need to notify other objects of the
change.  For example, when a prop gadget's pga_top value changes, its
target object(s) need to hear about it.

If an object needs to notify other objects about a change to one or more
of its attributes, it sends itself an OM_NOTIFY message.  The OM_NOTIFY
message will eventually end up in the hands of a superclass that
understands OM_NOTIFY and it will send om_update messages to the target
objects.