TrackMate_: ensure the 'Next' button is enabled after tracking process

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: optimize the spot feature grapher

For large models, it took extremely long to find the edges that belong
to the subgraph made of the selected spots. This commit proposes a
slightly faster method.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>