Display only the meaningful statistics

When running without the grayscale node, some statistics aren't meaningful
at all and crowd the interface. Those statistics are now displayed only
when the grayscale volume is present.
Also modified the export to follow make sure that what the user sees is
what he gets.
Finally, the possible statistics have been factorized in the logic to
simplify the maintenance of it. The chart options are automatically filled
with the relevant option after the statistics are computed

SQUASH WITH grayscale option
Add color name

Add the color label name as well as the column name (i.e. "Color").
Add "Percentage of volume" statistic

This statistics express the percentage of the image volume occupied by
a label.
Refactor ignore background

Previoulsy on could only hide the 0 label in the chart view. This change
allow the user to specify what label is to be considered background before
the statistics are run. If the background is ignored then the statistics
are only run on the rest of the image.