Add code generation for World subscriptions

[DagobertDev]

Currently many of my AEntitySetSystems use world subscriptions (like World.SubscribeComponentAdded or World.SubscribeComponentRemoved) to handle logic thats related to the system, but doesn't belong in the Update method. Because the subscription happens in the constructor, constructors are needed for many systems, where otherwise the generated constructor would be enough. Having an attribute to automaticially subscribe a method on world events would be very helpful.

[Doraku]

That's a good idea, using something similar to SubscribeAttribute for IPublisher.Subscribe completely slipped my mind when I added those apis.