Insert a bar to modify the transparency of the intensity level

[amichelini]

The transparency of the intensity layer is currently pre-set. It would be good that the user could set the level of intensity on its own. The intensity bar should be included into the pop up menu on the upper right. Alterantively and for the time being, it might be good just to decrease the level of intensity so that there is much richness of colors since not they see a bit too dim.