Move flag 'separateStructure' from Task to Workflow

[solth]

With the current structure of Kitodo.Production 3, all tasks in a workflow must have the same value for the flag separateStructure. This makes saving the flag for each task individually redundant.

For checking conditions in the MetadataEditor whether the combined or the separate structure tree should be shown, the editor therefor also needs to maintain a handle on the currentTask of a process, which unnecessarily complicates matters.

Because of this, the flag should be moved from the Task object to the Workflow object.