GtkButton child-displacement-y

["Boncek, John" <jboncek Hunter COM>]

Using GTK 2.2.4.  GtkButton provides the following property, among others:

Style Properties
"child-displacement-y" (gint : Read)
How far in the y direction to move the
child when the button is depressed.

Ibut says nothing about how to set it.  Apparently you can't do it
directly with g_object_set.  I tried that despite the fact it's marked
Read -- no effect.