[DMN Designer] DMNDiagram properties

The DMNDiagram node needs to expose the following properties (from etirelli on DROOLS-2870)

The Definitions properties that I think we need to expose are:

• Id (probably read only)
• name (needs to be editable; can default to the file name but should remain editable)
• namespace
• Also, the "description" element should probably be exposed somehow, although with lower priority. If you want just the minimal set, then the above.

Manual acceptance test

1.1 file

• Import model with namespace
• Import model without namespace

1.2 file

• B&D new model, non default namespace
• Import model with namespace
• Import model without namespace