Display color/false color composite images

[godber]

We need a way to select which three images in our ImageSet will be Red, Green and Blue and a way to toggle between RGB and grayscale.

We need to deal with multiband images like merview does. We need the ability to make color or "false color" images from three arbitrary bands (like we do with Pancam L257). We need to see "false color" from three grayscale images.

[godber]

Nice work @pbvarga1 !