Track accumulated intensity histogram.

Following the input of Robert

The specialized spot overlay can now display a histogram. This histogram is
built by collecting intensities of the spots belonging to the track.
So that the first spot of the track has only 1 count, the second spot 2, etc...

So we have a feature that is
- non scalar;
- stored for spots;
- but calculated once we have tracks.
Generalizing this feature will be difficult.
Signed off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>