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.
To not instantly free GUIFormSpec upon close/quit, Game periodically
cleans up the remaining instance on the next frame.
When a new formspec is received and processed after closing the previous formspec
but before the cleanup in Game, the formspec would be closed regardless.
This now re-creates the formspec when the old one is already pending for removal.
