code: func (ps *PrintSettings) SetDouble(key string, value float64) {
  	cstr := C.CString(key)
  	defer C.free(unsafe.Pointer(cstr))
  	C.gtk_print_settings_set_double(ps.native(), (*C.gchar)(cstr), C.gdouble(value))
  }
docs: // SetDouble() is a wrapper around gtk_print_settings_set_double().