MaMuT_" do not auto-track when there is no spots in selection

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: remove confusing method in TrackScheme friends

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: fix a compiler complaint on types

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: make theFiji menu item point to the right class

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: do not crash when TrackMate is launched with no image

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