TrackMate_: protected the NearestNeighborTracker against empty frames

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: do not crash when we modify a spot that does not belong to a track

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: prevent NPE when generating track colors

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: add a manual tracker item, that does nothing

But at least, it does not replace the current annotation in the GUI.
Still some bug: the track and edge analyzers do not get registered when
we pick this one.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>