Add: Asynchronous file loading

[Razish]

Since we (should) use handles for all loaded resources, we should provide the ability to load from disk asynchronously, which will be the default behaviour.
Once the resource is loaded, it should also have a callback so the loader/caller can read data from it (e.g. model dimensions, material information). This may be implemented by polling at the start or end of each frame so it can be done on the main thread.

ref #29 "Move mesh loading outside of renderer"
ref #41 "Generic resource manager"
depends #51 "Renderer + asset reloading"