Cleanup sound manager class (#7158)

* Cleanup sound manager client

* Use some const refs
* Use auto on iterators
* Drop unused parameters
* Move sound_openal.* to client folder
* Move sound.cpp + OnDemandSoundFetcher to client/ folder + reorganize includes properly

src/client.h

@@ -48,6 +48,7 @@ class MapBlockMesh;
 class IWritableTextureSource;
 class IWritableShaderSource;
 class IWritableItemDefManager;
+class ISoundManager;
 class NodeDefManager;
 //class IWritableCraftDefManager;
 class ClientMediaDownloader;