Stunner - [BPMN] In case task's actor is invalid, the resulting BPMN file is also invalid

Stunner sometimes generates invalid XML (according to the BPMN2 XSD at least).

• A user task with an actor assigned is generated as a potentialOwner element in the XSD
  <bpmn2:potentialOwner id="5058b718-8866-4ccd-b793-d5ebf48de5a2">
  <bpmn2:resourceAssignmentExpression id="_gN_feI4_Eem2-an8Fwu06w">
  <bpmn2:formalExpression id="_gN_feY4_Eem2-an8Fwu06w">manager</bpmn2:formalExpression>
  </bpmn2:resourceAssignmentExpression>
  </bpmn2:potentialOwner>

The id of the potentialOwner element is missing an underscore at the beginning (like other ids), which means that if by accident the first element is a number, this is now an invalid ID, we should always add _ as first element

• Tasks that have ioSpecification should always define an inputSet and outputSet, even if those are empty (there is no input mapping or output mapping). This can for example be reproduced by creating a user task with no output mapping. This will generate
  <bpmn2:ioSpecification id="_rEnOQY5AEem2-an8Fwu06w">
  <bpmn2:dataInput id="38B29C0C-6BD8-4275-ACCE-D5D97DEBB2CB_TaskNameInputX" drools:dtype="Object" itemSubjectRef="_38B29C0C-6BD8-4275-ACCE-D5D97DEBB2CB_TaskNameInputXItem" name="TaskName"/>
  <bpmn2:dataInput id="38B29C0C-6BD8-4275-ACCE-D5D97DEBB2CB_SkippableInputX" drools:dtype="Object" itemSubjectRef="_38B29C0C-6BD8-4275-ACCE-D5D97DEBB2CB_SkippableInputXItem" name="Skippable"/>
  <bpmn2:inputSet id="_rEnOQo5AEem2-an8Fwu06w">
  <bpmn2:dataInputRefs>_38B29C0C-6BD8-4275-ACCE-D5D97DEBB2CB_TaskNameInputX</bpmn2:dataInputRefs>
  <bpmn2:dataInputRefs>_38B29C0C-6BD8-4275-ACCE-D5D97DEBB2CB_SkippableInputX</bpmn2:dataInputRefs>
  </bpmn2:inputSet>
  </bpmn2:ioSpecification>

There is no outputSet on the above result, but there should always be one (and empty one).