Add Looping support for Sound objects.
Tested with the GStreamer backend. pygame and SDL backends still need testing.
Forgot to commit the pygame backend
Because the pygame and sdl audio backends use Clock callbacks to check for when the sound finishes
care must be taken to avoid running .play() inside ._check_play(). Schedule it with Clock.schedule_once
Merge pull request #1196 from bob-the-hamster/sound-looping

Sound looping