Implement Extensibility Part 2:
* Fix and extend the metamodel at some places, e.g. ExtendedMetadata was missing for all enums
* Add new MM attribute Extensions.xsdDefinition that receives the QName for the extension XSD element
* In XmlHandler resolve this URI via xsi:schemaLocation and covert the schema lo temporary Ecore on the fly
This allows sub classing extensions to be read and written.
Still missing:
* Unit test
* Correct save of "untyped" extension. In the moment they look strange.
* Transformation to XMI extensibility model.
32 files changed
