world.go
func (w *World) AddSystem(system System) {
	if initializer, ok := system.(Initializer); ok {
		initializer.New(w)
	}

	w.systems = append(w.systems, system)
	sort.Sort(w.systems)
}
func (w *World) AddSystem(system System) {
	if initializer, ok := system.(Initializer); ok {
		initializer.New(w)
	}

	w.systems = append(w.systems, system)
	sort.Sort(w.systems)
}
// AddSystem adds the given System to the World, sorted by priority.