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if the asset is already loaded, the calling listener is immedietly sent an assetLoaded(kFinished) event - otherwise the load is requested, and appropriate events are sent to indicate the result
the idea here is to make sure that any user object only receives the load event once, but still inform all objects that need to know as soon as the load occurs.
if the asset is already unloaded, the calling listener is immedietly sent an assetUnloaded(kFinished) event - otherwise the routine first asks all registered listeners whether they mind the asset unloading, and only completes the unload if they all agree