Support integration of Graphiti and Xtext (serialization)

Currently we are only able to use domain models that are pure EMF based. For 
supporting serialization of Xtext while editing and saving the Graphiti model 
some additional adaptions and checks are necessary as e.g. it is not possible 
to save an empty Xtext DSL model but it is per se possible to save/serialize an 
empty EMF model resp. an empty Graphiti model.

references:
 * http://www.eclipse.org/etrice/ (Graphiti - Xtext integration)
 * https://github.com/jeeeyul/graphiti-overview (uses Xcore and Graphiti)
 * my own prototype: https://github.com/joergreichert/Permet

Original issue reported on code.google.com by de.abg.r...@gmail.com on 21 Jul 2013 at 7:38

Blocking: #289

dvdkruk / spray

Support integration of Graphiti and Xtext (serialization) #288