Review feature shading/filtering enhancements for 2.11.1

JAL-2816 highlights a slight loss of functionality when configuring a linear shading scheme - that it it is now impossible to set a linear shading scheme (e.g white to black) for just a specific range of values within a wider set of data.

We also need to introduce a way to construct more complex interval shading schemes - the simplest might be to adopt colorbrewer conventions and allow categorical shading schemes to be applied based on a definition of intervals. Filters presents an opportunity to begin to construct these kind of sophisticated views, but doesn't go quite far enough.