* audout:u: Implement SetAudioOutVolume and GetAudioOutVolume
- Implementation of `audout:u` SetAudioOutVolume and GetAudioOutVolume (checked with RE).
- Add Get and Set for Volume into audio backends.
- Cleanup of all audio backends to follow the `IAalOutput` structure and .NET standard.
- Split OpenAL backend into 2 files for consistency.
* Address comments
* Fix the volume calculation
