/// <summary>
/// Adds invoker for level up event
/// </summary>
/// <param name="invoker">an invoker for level up event</param>
public static void AddLevelUpInvoker(HUD invoker)
{
levelUpInvokers.Add(invoker);
foreach (UnityAction <int> listener in levelUpListeners)
{
invoker.AddLevelUpListener(listener);
}
}