Window/Level auto selection based on ROI

[aylward]

Should be able to select an ROI and have the histogram and color/opacity functions updated to match the distribution of data within that ROI.

Thanks to Steve Pieper for the suggestion.

[thewtex]

This is a great idea. Since we already have the ability to select an ROI, we could have a button that adjusts the color range min/max based on the current ROI.