TrackMate_: make the action provider stateles

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: make the export action more easily reusable.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: deal gracefully with ImagePlus created ex nihilo

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: blank constructor for DetectorProvider

Simplifies API calls. A lot.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: use a blank constructor for TrackerProvider

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