An arrow pointing in one of four directions. The 'etched' shadow
types does not work.
GTK.Arrow(GTK.ARROW_UP, GTK.SHADOW_OUT)
GTK.Arrow(GTK.ARROW_LEFT, GTK.SHADOW_IN)
GTK.Arrow(GTK.ARROW_RIGHT, GTK.SHADOW_IN)
GTK.Arrow(GTK.ARROW_DOWN, GTK.SHADOW_OUT)