TrackMate_: the tracks can be navigated through  using the keyboard in TrackScheme

- UP: previous spot in time.
- DOWN: next spot in time.
- LEFT: next sibling within the same track
- RIGHT: previous sibling within the same track
- PAGE-DOWN: jump to next track, same frame
- PAGE-UP: jump to previous track, same frame
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>