TrackMate_: accept XML from v >= 2.0.0

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: protect the 3D TrackMate viewer against empty frames

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackMate_: optimize startup of the 3D viewer a bit

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Merge remote-tracking branch 'origin/master' into track-mate

Conflicts:
	src-plugins/TrackMate_/src/main/java/fiji/plugin/trackmate/visualization/threedviewer/SpotDisplayer3D.java

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

Convenience methods to return an iterable view of a spot collection.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>