Issue #1982: Fix RGB histogram pixel/count values

This attempts to resolve #1982 (closed).

In the Histogram dockable, choosing the RGB channel overlays the Red, Green, and Blue histograms over each other. This also triples the pixel and count values (since it's counting all three histogram values at once). This patch resolves the issue by dividing the result of gimp_histogram_get_count () for the Pixel and Count displays by the number of channels.

@Jehan Hi! When you have time, is this the desired behavior? E.g. Should the RGB channel show the actual number of pixels (this patch) or the sum of pixels in all three histograms (the current behavior)?