[1.4] Redesign assets to be self-sufficient.

[Mirsario]

For 1.4 TML, we should really redesign the new asset system that ReLogic added to their library. Currently, assets don't serve much function besides just wrapping objects and describing their state. They do not trigger any loading processes when accessed, and content assets still require the user to manually call methods like Main.instance.LoadItem(int), which call AssetRepository.Request<T>() with an ImmediateLoad parameter instead of the initial DontLoad.