[Paint] enhancement of Image Statistic Mode as combobox

[mathildemerle]

In Paint Segmentation toolbox, in Magic Wand, there are 2 modes: a manual one, and an "Image Statistic" one.
Using the first or the second totally change the way the region growing works.

It could be interesting to emphasize the 2 modes using a combobox instead of a checkbox for the switch between both.