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/unittest/test.cpp

@@ -19,6 +19,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 #include "test.h"
 
+#include "client/sound.h"
 #include "nodedef.h"
 #include "itemdef.h"
 #include "gamedef.h"