Release v2.4.0

- TrackScheme now has an outline panel, useful to navigate in large models.

- TrackScheme has a better layout for tracks that have merge and split events.

- The 'Refresh' button on detector config panels is not present anymore. It's
confusing and useless now that the quality value does not match pixel intensity
at all.

- TrackMate has new logos and icons, made by Agnieszka Kawska @ IlluScientia.
http://www.illuscientia.com/

- Fixed several problems with the 'Copy Overlay' action.

- Fixed a severe TrackScheme bug, happening when creating links with invisible
tracks.

- Fixed a detector crash when running a preview detection with 0 spots found.

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