feat(import/BpmnTreeWalker): make businessObject#di prop configurable

This allows advanced use-cases such as lazy importing
elements into the diagram.

lib/import/BpmnTreeWalker.js

@@ -10,8 +10,10 @@ import {
   elementToString
 } from './Util';
 
-// TODO: should be configurable: true as well which would allow to remove binding
+var diRefs = new Refs(
-var diRefs = new Refs({ name: 'bpmnElement', enumerable: true }, { name: 'di' });
+  { name: 'bpmnElement', enumerable: true },
+  { name: 'di', configurable: true }
+);
 
 /**
  * Returns true if an element has the given meta-model type