Support collaboration models without a default process

[rsoika]

BPMN model generated with bpmn.io may miss a default process in the collaboration definition. This leads to an invalid model structure.
We can fix this issue by auto detecting these cases and add a default process on the fly. In addition we should set the isDirty flag to true to indicate the user the new added process element.

Further more also TextAnnotations can be part of the collaboration node instead of the default node in a collaboration model created with bpmn.io. Also this case should be fixed automatically by assigning the TextAnnotation node to the default process.