Code style changes.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackScheme now offers two levels of background decorations.

- 0: nothing;
- 1: the track names and lanes
- 2: 1 + the row names and alternating color.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Merge remote-tracking branch 'origin/trackscheme-background'
Fix and simplify loading/saving API.

We do not need the providers to save and load anymore.
The tracker or detecotr keys are handled by the mother TmXmlWriter
class. The concrete factories just have to marshal their specific
parameters.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Reactivate and rebrush the IcyExporter.

To acquaint the recent work Fabrice (De Chaumont) did in Icy to have
this importer work without problems.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>