Fix a bug that prevented the XML file to be saved.

Because we are now playing wit feature analyzer that can be manual,
there can be null values for features. They should not forbid a file to
to be saved.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
When graphing spot feaures, only include spots from visible tracks.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Release v2.5.4.

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