TrackMate_: code style change.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: add a method to provide functionality in the GUI,

when the user navigates backward.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: code style changes.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: add clear() to SpotCollection.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: code style changes.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: add methods to clear the tracks of the model.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: navigating backward clears the model.

- Navigating backward to the detector configuration panel clears the
  spots.
- Navigating backward to the tracker choice panel clears the tracks.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>