Add Strength parameter to split-toning tool

Four settings is a lot, but it's still better than Adobe Lightroom's
five sliders... right?  ;)  Actually, I think Lightroom has their
saturation sliders control two things: blend strength, *and* saturation
of the mixed color.  That strategy would explain why they get no
noticeable change in pixel color when saturation is 0, and how they
avoid use of an explicit "strength" slider.

We could do something similar in PD, but for now, I think the current
setup is pretty intuitive.