Histogram improvements

This PR has a few improvements I made will trying to examine the histogram for an image.  Specifically, it does the following:

* Adds a check box that allows the y-value of the histogram to be the log of the pixel value instead of the pixel value itself - this is *crucial* to use with "full image", as otherwise the histogram is almost always all in one huge bin near 0.
* Adds a check box that changes the x-limits of the histogram to span the range set by the low/high cut levels.  This seemed like the easiest way to "be able to "zoom" in on a part of the histogram without adding more UI elements than necessary.
* Changes the plotting of the histogram to be "steps" instead of a line plot, as a histogram shouldn't actually be lines anyway.  Note that this requires some care to make sure matplotlib renders the step correctly (rather than off by 1 in x), but I think I got it right.


Currently, this is only implemented on the QT side, because that was easy for me to test, but if this looks good to you, @ejeschke, I can extend it to the GTK histogram plugin.