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/sound_openal.h

@@ -0,0 +1,31 @@
+/*
+Minetest
+Copyright (C) 2013 celeron55, Perttu Ahola <celeron55@gmail.com>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#pragma once
+
+#include <memory>
+
+#include "sound.h"
+
+class SoundManagerSingleton;
+extern std::shared_ptr<SoundManagerSingleton> g_sound_manager_singleton;
+
+std::shared_ptr<SoundManagerSingleton> createSoundManagerSingleton();
+ISoundManager *createOpenALSoundManager(
+		SoundManagerSingleton *smg, OnDemandSoundFetcher *fetcher);