Custom editors for DataView items

The basic implementation of data views relies on the underlying toolkit to provide different editors when editing an item based on the type of the item's editor value.

We would like to provide a hook where users can provide their own editors for items.

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

A plan for implementing this using an Enum/ComboBox value type as a pathfinder:

- [ ] add an Enum value type (#782)
- [ ] create basic editor hooks that allow the use of a ComboBox for Enum values (eg. a `pyface.field.combobox_field`)
- [ ] generalize to other editors (eg. bounds on spinboxes)
- [ ] provide some way to drop in a TraitsUI view