TrackMate_: make an entry point for manual tracking

This new plugin menu entry launch TrackMate GUI on a limited number of
panels, selected to be suitable for manual tracking.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: minor GUI polishing

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