Custom rendering for Data View items

The basic implementation of data views relies on the underlying toolkit to render items. We would like to provide a hook where users can provide their own renderers for items that completely overrides the defaults.

This would permit things like rendering plots or images in the place of standard cell contents.

This should be a feature of the view, rather than the model, so that different views of the same model can choose to render items in different ways (or not, as appropriate).