The bug concerns the image-scaled-ratio-max/min for image-mode. It was a
beginners error : dividing two integers that should provide a number
between 0 and 1 ...

- Cleaned the unused methods in J2DGraphRenderer (since the changes in the global Camera system).
- Renamed ElementInfo, NodeInfo and EdgeInfo, into Skeleton, AreaSkeleton and ConnectorSkeleton.
- Ensured the most parts of the rendering pipeline are swing-free. Only
  the shape system uses Swing. This will prepare things for distinct
  back-ends, one using Swing/Java2D the other using OpenGL.