Histogram and ClassBreakSlider

Displaying a histogram of the distribution of a feature attribute, and adding a slider to change class breaks.

The grid shows the current classes and their renderers. Class breaks can also be customized by changing the filters in the grid. Slider thumb positions will be updated to reflect the changes, and class breaks are aligned thanks to the GroupStyleReader.

Note that the js is not minified so it is readable. See histogram.js.