Make the Model extendable.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Make some components of TrackMate extendable.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Add the option for the spot overlay to draw an extra layer.

Via a protected method called after the normal spots are drawn.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
Code style changes.

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

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Merge branch 'master' of github.com:fiji/TrackMate