This adds a new setting to set sound volume multiplier when Minetest window is unfocused/inactive (sound_volume_unfocused, located in Settings > Graphics and Audio > Audio > Volume when unfocused).
If the window is not focused, the sound volume will be multiplied by sound_volume_unfocused setting. The sound volume will be set back to sound_volume again when the window is focused.
* 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