Overhaul load state machine

[ndubois-legion]

Summary

Leaner API & make loading steps more configurable per type

Current State

Currently, loading is klunkly. APIs should probably have a single entry point for accessing resources, in both offline & runtime. We should also have a way for defining the loading graph per resource type, or just per resource. Use Reference<T> in the APIs, and avoid the mix of Handle<T>, HandleUntyped, ReferenceUntype. Are those really useful?

Tasks

• [x] Remove all the AssetRegistry::loadXXX() APIs and keep a single async one.
• [x] Remove the loading thread
• [x] Implement entity
• [x] Implement an example, like shader and texture
• [ ] Review API with Rendering team ...