This increase the safety of our build; external consumers
do no longer need to account for the `browser` field to
bundle bpmn-js (or otherwise bundle a Node shim, unintentionally).
* copy references when copying element
* add referenced root element if it doesn't exist
* do NOT add referenced root element if root element with same ID exists
Related to camunda/camunda-modeler#1049.
Related to camunda/camunda-modeler#1463.