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.
Dropped ServerSoundParams -> moved to ServerPlayingSound. This gets rid of the duplicated
'fade' and 'pitch' values on server-side where only one was used anyway.
SimpleSoundSpec is the basic sound without positional information, hence 'loop' is included.
Recursively added PROTOCOL_VERSION to most functions to reduce the versioning mess in the
future. Per-type version numbers are kept for now as a safety rope in a special case.
* 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