sound applet:  Fix a warning, and improve track position accuracy:

- timeout_add_seconds isn't 100% accurate, but instead of switching to
  timeout_add (ms), we can keep what we have currently, and occasionally
  re-sync the position from the player itself.  It's still imperfect,
  but it won't require additional processing resources, and at worst
  it should only ever be 1 second off.