audout:u: Implement SetAudioOutVolume and GetAudioOutVolume (#781)

* 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

Ryujinx.Audio/IAalOutput.cs

@@ -15,8 +15,13 @@ namespace Ryujinx.Audio
         void AppendBuffer<T>(int trackId, long bufferTag, T[] buffer)  where T : struct;
 
         void Start(int trackId);
+
         void Stop(int trackId);
 
+        float GetVolume();
+
+        void SetVolume(float volume);
+
         PlaybackState GetState(int trackId);
     }
 }