TrackMate_: fix a bug when adding a single spot to a track

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: add a plugin to load a TrackMate file

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