This method tells an object to set one or more of its attributes.
Applications should not call this method directly.  Instead, use the
intuition.library functions SetAttrs() and SetGadgetAttrs() to call this
method.  The return value for this method is not explicitly defined.

The return value for this method is not explicitly defined.  However, in
general, when implementing the OM_SET method, if setting an object
attribute causes some sort of visual state change, the OM_SET method
should return a value greater than zero.  If changing an attribute does
not affect the visual state, OM_SET should return zero.

This method uses a custom message (defined in <intuition/classusr.h>):

    struct opSet {
        ULONG             MethodID;      /* OM_SET */
        struct TagItem    *ops_AttrList; /* tag list of attributes to set*/
        struct GadgetInfo *ops_GInfo;
    };

The ops_AttrList field contains a pointer to a tag list of attribute/value
pairs.  These pairs contain the IDs and the new values of the attributes
to set.  The dispatcher has to look through this list (see docs for the
utility.library NextTagItem() function) for attributes its class
recognizes and set the attribute's value accordingly.  The dispatcher
should let its superclass handle any attributes it does not recognize.

If the object is a gadget, the ops_GInfo field contains a pointer to a
GadgetInfo structure.  Otherwise, the value in ops_GInfo is undefined.
Intuition use the GadgetInfo structure to pass display information to
gadgets.  See the gadgetclass methods for more details.