Workflow editor needs to be able to handle step="-1" for common-actions

Osworkflow allows to configure common-actions with no transition by setting the step attribute equal to "-1". See the osworkflow docs on this feature. Currently the workfloweditor does not support actions that don't result in a transition. The transition screen still displays an arrow pointing at an empty destination step.

However, the XML import of the workflow does not restrict the user from creating a workflow with step="-1". This results in exceptions thrown in log file when viewing the workflow in the editor, although the workflow will work.