callback for left, right arrows

[Brent Pedersen]

when pressing the left/right arrow in the flash movie, the flash should call the javascript functions

Gobe.left(), Gobe.right()
which take arguments track_id, current_start, current_end. where track_id indicates the track under the mouse cursor

these functions can then do something like Gobe.set_data() with new extents for the track_id in order to implement panning.