Add a GameplayMessage system to resolve hard-coupling of the AttributeComponent and the GameMode

[rolandsarosy]

In CAttributeComponent.cpp:

	if (!IsAlive())
	{
		MulticastOnDeath(InstigatorActor);

		// I greatly dislike this hard-coupled nightmare of a statement, but this is what was done in class. TODO: Use something like a GameplayMessage or maybe a delegate of sorts here.
		GetWorld()->GetAuthGameMode<ACGameModeBase>()->OnActorKilled(GetOwner(), InstigatorActor);
	}