RFC: Switch many loaders to asynchronous

This allows the loaders to not block other actions while they are loading, improving startup speed and general item creation speed.

From profiling, this reduces the startup time of Koko by about 500ms, with the best case I have seen now being 600ms until first frame. The drawback is that this first frame will not necessarily show anything useful, so you get a frame where there's only background, then the UI pops in a little after that. Personally I think it looks fine, but others may disagree.