Values : take an optional `getValues` function and make `values` a getter to fix #43

SelectEditor : add a private `_refreshValues` and use it to call
`values` on the annotation each time `refreshElement` is called.
FieldView : in `_refresh` call `refreshElement` on the editor

This allows elements to refresh when the user calls `refresh` on an
ObjectEditor.  One use case is when the user knows the possible `values`
for a `SelectEditor` have changed and wants to call this to refresh the
editors for the active selection in the object editor.