Reformat Layer Controls for RGB composites [1356]

See Inq for details - there was some confusion because Apply button did not apply the common gamma.  
Here are before/after pics of the layout:

Before:

![rgblayercontrolsold](https://f.cloud.github.com/assets/2334847/862871/2cdc9d5e-f5fd-11e2-955f-6dea5b399964.png)

After:

![rgblayercontrolsnew](https://f.cloud.github.com/assets/2334847/862873/3a73a066-f5fd-11e2-8a63-52d9a67f4e0d.png)