[DMN Designer] Add support for optional typeRefs

At the moment TypeRef is a mandatory field within the editor.

This is a hard limitation of use of the Properties Panel bean-binding (it cannot bind to null values). However the DMN Specification states that TypeRef are optional (and we are working around by changing the default to Any - see DROOLS-3248).

We need to change the UI and marshaller to be able to support optional TypeRef. This would (probably) require adding a magic value to BuiltInType representing a "null" selection. It would need to:

• Appear at the top of the "Data Type selector" widget (with label "<None>" or similar)
• Be the selected option when the TypeRef is the new magic value (this should happen automatically, but worth checking!)
• The marshaller should set the TypeRef to the magic BuiltInType if the DMN model (coming from XML->kie-dmn-model->workbench model) has a null TypeRef - the magic value is only to be used client-side.
• The marshaller should set the TypeRef to null when converting from the workbench model to kie-dmn-model and the workbench model TypeRef is the magic value - again, the magic value is only to be used client-side.
• Check that creating new data-types does not show the magic value (it uses a different widget, but population of the widget probably iterates all BuiltInType.

mmortari@redhat.com karreiro_ seem reasonable to you?