Utility loaders to asynchronously create texture at launch and manage it correctly

Another proposal for #2049 which encapsulate Texture derived class inside loader to avoid confusion.
Those derived classes are only usefull when registering loaders and generates no conflict with TextureLoader...

What do you think ?
Also, it should return Texture, not FreePaintTexture. I elaborate on how to make that work with the normal TextureLoader in the discussion section of this PR.