Add methods to directly load assets from World

[nicopap]

Objective

FromWorld is often used to group loading and creation of assets for resources.

With this setup, users often end up repetitively calling .resource::<AssetServer> and .resource_mut::<Assets<T>>, and may have difficulties handling lifetimes of the returned references.

Solution

Add extension methods to World to add and load assets, through a new extension trait defined in bevy_asset.

Other considerations

• This might be a bit too "magic", as it makes implicit the resource access.
• We could also implement DirectAssetAccessExt on App, but it didn't feel necessary, as FromWorld is the principal use-case here.

---

Changelog

• Add the DirectAssetAccessExt trait, which adds the add_asset, load_asset and load_asset_with_settings method to the World type.

[valaphee]

There is also the downside that it generates more code/or is slightly slower as it has to do a lookup each time you add an asset.

But it should be a "cold" anyway.

[afonsolage]

The examples looks a lot cleaner and more intuitive, I like it

[alice-i-cecile]

I like the diff in the examples a lot, and I'm pleased with the code quality here.

It is more magical, but this method will also be more robust to any future changes in how asset loading works.