WIP migrate to 0.12

[mogambro]

This is a very, very, very rough transition from bevy 0.11 to 0.12.

As assets v2 dropped with this release a lot of API surface changed.

The biggest problem for me were and are:

1. LoadContext is now needed to be a mutable reference for quite a few calls and that doesn't play nice with async.
2. Transitioning between AssetId and Handle. One can upgrade an AssetId to a strong Handle via asset_server.get_id_handle(id) but I don't have a access to an AssetServer in the AssetLoader load method.