* Improved histogram display in Layers->Histogram. Show multiple
colors.
* Added histogram docker widget.
* Improved Ko*HistogramProducer classes. They were not handling
displaying different color space histograms correctly - data was
just converted to different bit depth. Histogram data has to be
converted to the requested color space for the histogram to make
sense.

Patch by Eugene Ingerman. Thanks!
Phabricator diff: D1766