improve ROI setting

## Use Case

The ROI volume clipping sliders are too imprecise - I want to know what pixels/slices I am clipped to.
I also want to be able to clip some channels and not others.

## Solution

ROI x,y,z sliders for each channel, plus one global set.
Write numbers on the slider handles and the ends so I know where they are set and what the range of values is. Maybe even add the ability to key in exact numbers.

Bonus: interactive way to move the ROI in the viewport