Gamma and Saturation slider for Composition Panel

[Shebbe]

After the appericiated revert of the Exposure Slider value box instead of a dropdown I think we can expand the feature set even more for compositors. There's plenty of space left.

I'd like to see a Gamma and Saturation slider too. It is very common to adjust these for checking comps.

Their values should be 1,0 default meaning no change.

Personally I would also prefer the exposure slider to be 1,0 at base because it's math is actually just multiplication, it isn't really stops in + or -. Doing 2,0 to add a stop makes more sense because we want to multiply the light by 2. 0,5 for 1 stop down etc.