Merge: app::audio: remove deprecated services and other minor improvements

This PR removes the old `load_sound` and `load_music` services, they were replaced by lazy loading services a while back. The new services are more in line with the other asset loading services from `gamnit`.

There are also other general but minor improvements: doc updates, better names for a param and an attribute, less warnings, less casts, more consistent error messages and easier debugging.

Pull-Request: #2300
Reviewed-by: Jean Privat <jean@pryen.org>