private void DoneDisplayingAchievement(AchievementNotifier sender)
{
_achievementNotifications.Remove(sender);
}
private void AchievementUnlocked(AchievementNotifier achievement)
{
achievement.DoneDisplaying += new AchievementNotifier.AchievementNotifierEvent(DoneDisplayingAchievement);
_achievementNotifications.Add(achievement);
}