TrackMate_: deal with GUI buttons correctly.

We have to delegate re-activating some button states to the descriptors
themselves, if we want to properly prevent the user from moving to
invalid GUI states.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: fix the plot Nspots vs time action

A NPW was thrown for frames without any spots: we needed at least one
spot to determine what was the POSITION_T at this frame.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>