InstanceEditor should use standard traits adaptation rather than rolling its own

Traits already has methods for adapting values from one type to another. We should use those.

This would likely require having the `InstanceEditor` have its own adaptation manager (with a sensible default) and some refactoring of `InstanceChoice` as an `Adapter`.

Low priority, but potentially powerful because it allows multiple adapters to be used, with a standard way of selecting them.