Reconsider the structure of separated eca config entities

Just realized that there is a use case, which needs to be considered when restructuring that: when sharing ECA models with other Drupal sites, or deploying them from one stage to another of the same Drupal site, config management does the job as everything related to the ECA model lives in config. If we were storing the proprietary data somewhere else, it wouldn't go with the model in that context, which is bad.

With the latest bpmn_ui modeller, we can now upstream convert ECA models into the bpmn_io canvas without even having any eca.model.* file present at all. Not all properties from BPMN are available yet in the ECA model, but we could bring across more meta data to make it complete. Then, if we want to rely on the auto-layout function of BPMN alone, we could (optionally) turn off the XML data storage.