Good practice with Swing.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
TrackScheme: tooltip for row and column headers.

Hovering the mouse there shows a tooltip that gives either the target
frame or the target track name. Useful when you are very unzoomed and
need to know where you are.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>