Allow derived BPMN models to be edited

Hi,

Sometimes in clients they require specific custom models that are very similar to BPMN but with a few modifications (adding one objecto/custom object, connection,...). In other models like EPC, this is simple since it is possible to just derive the model and then edit it, but all the BPMN models are restricted even when derived.

This makes customizing BPMN models very hard since it is needed to create a new one from scratch and define all the objects needed and what is worse, all the connections possible between those objects.

Therefore I suggest the same behaviour present from EPC models in which you can derive them and use them as a base for the custom model, is replicated in BPMN models.