Advtrains Sounds API 1.0

Adds various event sounds of wagons (arrival, departure, driving and etc) for advtrains and API for their registration. It provides the new table field for advtrains.register_wagon in the following form:

sounds={  -- listing ogg files
  door_open=<soundname>,     -- plays on doors opening
  door_close=<soundname>,     -- plays on doors closing
  depart=<soundname>,              -- plays when train speed is increasing (departure)
  arrive=<soundname>,                -- plays when train speed is decreasing (arrival) 
  loop<soundname>,                      -- plays when train is going at constant speed
  loop_inside=<soundname>,    -- plays when train is going at constant speed and player is inside it
  loop_outside=<soundname> -- plays when train is going at constant speed and locating from player at the distance of 20 blocks
}```