Update to Bevy 0.12

[luca-della-vedova]

New feature implementation

Implemented feature

Closes #192, #128. Update to Bevy 0.12.

Implementation description

Apart from the usual changed APIs, Bevy 0.12 introduced a virtual rewrite of the asset system that allowed a significant refactor and cleanup of the way we handle assets.

Specifically, the most important feature is support for multiple asset sources, which allows us to split the sources into separate structures, with two main consequences:

• The asset source logic is a lot more modular, external plugins can register their sources without having to edit a monolythic piece of code.
• The AssetSource enum is a lot cleaner, we can split out the sources that don't necessarily make sense for models (i.e. Bundled, used for icons and OSMTile used for open street map tiles) and put them in their own asset source.

Specifically for Bundled assets, the new asset system introduced a builtin embedded:// asset source that does exactly the same so we can get rid of more code.

Because of #128, it was necessary to move out of surf. Luckily a new crate came up recently, ehttp which is actively maintained, does the job perfectly fine and is very lightweight (our dependency count decreased by about 100 crates by switching!)

[luca-della-vedova]

There is currently a regression in workcell editor mode where the cursor is always displayed, currently investigating

[luca-della-vedova]

This PR is now ready for review.

Since this migration added a lot more deprecation warnings with the deprecation of event.iter() I took the chance to clear out all the trivial warnings from the codebase so we can avoid introducing new ones (they would get drowned in the 100+ existing warnings). The only remaining ones now are nameclash collisions (also introduced here, although not breaking, for now) and one unused field that sounded not trivial.