code: func (ps *PrintSettings) ForEach(cb PrintSettingsCallback, userData uintptr) {
  	printSettingsCallbackRegistry.Lock()
  	id := printSettingsCallbackRegistry.next
  	printSettingsCallbackRegistry.next++
  	printSettingsCallbackRegistry.m[id] =
  		printSettingsCallbackData{fn: cb, userData: userData}
  	printSettingsCallbackRegistry.Unlock()
  
  	C._gtk_print_settings_foreach(ps.native(), C.gpointer(uintptr(id)))
  }
docs: // Foreach() is a wrapper around gtk_print_settings_foreach().