make the artifacts element support flowElements

[evin2333]

I have customized an element custom:Group, which inherits bpmn:Group and contains a new property flowElements:FlowElements.
When I finished saving the data and reloading the xml, I found that the child element under this custom:Group was not rendering.

[nikku]

@evin2333 I checked against the BPMN 2.0 meta-model and found that bpmn:Group does not support flowElements. Am I mistaken?

We'll not accept any contributions to add custom extensions in the core. Instead I recommend you to build your custom version of our BPMN editor to support your extensions there.