new icons for the Artistic color selector

change the Toggle gamut mask button to be disabled when no mask is set,
so it keeps showing the informative message "Select a mask in the Gamut Masks docker";
and only enable it to toggle to "Mask is disabled" when a mask is actually loaded.

new configure-thicker icon, used in several dockers.
I'm keeping the other thinner configure icon in a few places where it fits better.