TrackMate_: rework trackscheme toolbar

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: in TrackScheme, thumbnails are now optional

The user can toggle thumbnail capture on/off.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>