code: func (ps *PrintSettings) Set(key, value string) {
  	cKey := C.CString(key)
  	defer C.free(unsafe.Pointer(cKey))
  	cValue := C.CString(value)
  	defer C.free(unsafe.Pointer(cValue))
  	C.gtk_print_settings_set(ps.native(), (*C.gchar)(cKey), (*C.gchar)(cValue))
  }
docs: // Set() is a wrapper around gtk_print_settings_set().
  // TODO: Since value can't be nil, we can't unset values here.