This attribute points to a tag list of attribute mappings which the gadget
uses to change the attribute IDs of an OM_UPDATE's attribute/value pairs.
For example, if a gadget had the following ICA_MAP:

    struct TagItem map[] =
    {
        {PGA_Top, STRINGA_LongVal},
        {MYATTR, MYNEWATTR},
        {TAG_END, }
    };

before it sends an OM_UPDATE to its ICA_TARGET, the gadget scans through
the OM_UPDATE message's attribute/value pairs looking for the PGA_Top and
MYATTR attributes.  If it finds the PGA_Top attribute, it changes PGA_Top
to STRINGA_LongVal.  Likewise, if the gadget finds the MYATTR attribute,
it changes MYATTR to MYNEWATTR.  The gadget does not disturb the
attribute's value, only its ID.