Fix schmaeditor to preserve i18n Message values

A couple of todo to merge:

- fix to always edit the default value (editing message id may not make sense)
- tests (that Message is preserved and wrapper does not have side effects)

And then there will be also a pull for master.

Yet, comments about the approach is welcome. It's not nice that I have to wrap a field to register a custom data manager for it, but could not figure out any simpler (and as generic) way yet.