Allow reuse of variable names in multiinstance tasks

[essweine]

When specifying a task data variable, we use the ID of the BPMN element. Since IDs are unique, duplicates get ignored when bpmn-js parses the XML. We should create item definitions for these variables and use references to these definitions instead. Thsi will entail

• modifying bpmn-js to generate the appropriate XML
• updating the spiffworkflow library to parse this XML
• implementing a migration strategy for existing XML files