This adds the possibility to show an histogram in the pqColorOpacityEditorWidget
When showed, the texture is hidden.
It is controlled by the following button :
A checkbox to enable/disable the showing of the histogram
A button that highlights when the histogram is out of date and compute the histogram when clicked
A checkbox to automaticaly compute the histogram if it is out of date
A slider to control the number of bins of the histogram

This also adds a test and a .md file.