Implement serialization support for i18n message

For discussion. Replaces previous i18n parser implementation so, do not merge yet.

See also: https://github.com/plone/plone.app.dexterity/issues/132

@davisagli 

I couldn't figure out a such nice way to implement i18n serialization support, which would have been similar to the current parser implementation, so I ended up implementing the support as a field metadata handler (plus a few lines to add the default domain into model root tag).

What do you think? Of course, I could also restore the original parser implementation so that these could coexist.