gtk/widget_since_3_8.go
func (v *Widget) SetOpacity(opacity float64) {
	C.gtk_widget_set_opacity(v.native(), C.double(opacity))
}
func (v *Widget) SetOpacity(opacity float64) {
	C.gtk_widget_set_opacity(v.native(), C.double(opacity))
}
// SetOpacity is a wrapper around gtk_widget_set_opacity()