[meta] Improve the accessibility of the BPMN.iO module

Thanks @mxh for the heads-up. I had raised that issue in Barcelona as well, when we considered ECA, and I suggested adding it without the UI for exactly that a11y reason. The feeling at the time had been, that hiding ECA in the black box and leaving the UI out would be more of an issue and the BPMN UI is considered important.

As that UI is being integrated from an external library, making that accessible is a task that needs to be addressed there anyway. Yes, we can tackle small parts of it in Drupal as well, but the major task is upstream and out of our control. The good news, however, they are aware and have it on their agenda. As I understand it, the issue is more about how to go about this in general, where BPMN models can contain hundreds of elements that a user needs to navigate through. And there doesn't seem to be an accepted method on how to address that. Once there is a conclusion about that, technical implementation is considered to be the smaller part.