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>
POM: add developers and contributors sections
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
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/main/java/fiji/plugin/trackmate/FeatureModel.java
	src/main/java/fiji/plugin/trackmate/LoadTrackMatePlugIn_.java
	src/main/java/fiji/plugin/trackmate/Model.java

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Prevent NPE when a track disappear before its velocity could be measured.

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