Dublin Core crosswalk output

[gregjan]

Add a crosswalk extension point for output of arbitrary XML schemas or models. Generate such an extension for Dublin Core XML and convert the MODS model plugin to fit this extension point as well.

[gregjan]

A stretch goal is to give the user the ability to generate output models based on an XML schema they provide. This would add a few runtime EMF plugins to the platform. It presents a few design challenges, namely whether to keep creating the output XML via ECore or to use a simpler DOM-style library plus some form of schema-based discovery.