code: func (v *Widget) AddTickCallback(fn TickCallback, userData uintptr) int {
  	tickCallbackRegistry.Lock()
  	id := tickCallbackRegistry.next
  	tickCallbackRegistry.next++
  	tickCallbackRegistry.m[id] = tickCallbackData{fn: fn, userData: userData}
  	tickCallbackRegistry.Unlock()
  
  	return int(C._gtk_widget_add_tick_callback(v.native(), C.gpointer(uintptr(id))))
  }
docs: // AddTickCallback is a wrapper around gtk_widget_add_tick_callback().