Sets the shadow around the button to
GTK_SHADOW_OUT until the button is pressed,
and GTK_SHADOW_IN when it is. This is currently
the normal relief style for a button.
2
GTK_RELIEF_NONE
Sets the shadow around the button to
GTK_SHADOW_NONE except when the cursor enters
the area of the button, meaning that only the contents (e.g. the label) of
the button show until that point.
