Add BPMN 2.0 Spec compliant InputOutputSpecification behavior

[nikku]

Is your feature request related to a problem? Please describe.

Right now we got a hack in place which ensures that we produce valid XML when users model data objects / data object references. This, however does not result in BPMN 2.0 spec compliant bpmn:InputOutputSpecification elements to be generated and maintained during modeling. To support interoperability with other tools we should get rid of our hack and add proper support for spec compliant input/output handling.

Describe the solution you'd like

• Maintain bpmn:InputOutputSpecification in a BPMN 2.0 spec compliant way
• Ensure that old diagrams (including our hack) can be converted on the fly to the new behavior
• Investigate transition strategy from old to new structure
• Create an integration test that verifies we always generate BPMN spec compliant XML during modeling

Describe alternatives you've considered

Keep things as is, at the cost of tool interoperability.

Additional context

Ongoing work already happened at #910.