Tells an object to report an attribute's value.  Applications should not
call this method directly.  Instead, use the intuition.library function
GetAttr().  The return value for this method is not explicitly defined.

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

    struct opGet {
        ULONG MethodID;     /* OM_GET */
        ULONG opg_AttrID;   /* ID of attribute to get */
        ULONG *opg_Storage; /* place to put attribute value */
    };

If the object's dispatcher recognizes opg_AttrID as one of the attributes
defined by this class, the dispatcher should copy the value of that
attribute to where opg_Storage points:

    struct opGet *myopget;
    ...
    *(myopget->opg_Storage) = my_attribute_value;
    ...

If the dispatcher does not recognize opg_AttrID, it should pass the
message on to the superclass.