Add a histogram view

[darbyjohnston]

It would be nice to have a histogram tool similar to DJV. Are there other tools that would be helpful for inspection and QA?

Also related:
#29
#31
#34

[ggarra13]

A Vectorscope is useful in showing the colors when they are not in the NTSC/PAL range. (mrv2 has that, so you can check how I do it).
A Wavefrom monitor is useful in checking the luminosity of each area across an image axis (it can be horizontal- the most common- or vertical). (ffmpeg has a custom filter implementation of a horizontal one, but it scales it as large as the image is without resizing it. Also I think it is GPL only and does not support float colors).