Allow `HistogramWidget` to show multiple layers at once

[dstansby]

Currently HistogramWidget can only show one selected layer. If multiple layers are selected, it should overlay histograms from each layer on the same plot, and generate a legend to identify each histogram.

[dstansby]

I had a play around trying to implement this, and ran into the issue of what to do when multiple RGB layers are selected,
or a mix of grayscale and RGB layers are selected. So I think unless anyone can come back with a suggestion of how to plot these two cases, I'm going to close this as something we won't implement.