Move saveDraft and editDraft from DocumentWorkflow to EditableWorkflow

Move #saveDraft and #keepDraft from DocumentWorkflow interface to the EditableWorkflow interface, because:

• it reduces the number of casts in workflow related code in the channel manager and the content editor ( The workflowManager returns a workflow of type EditableWorkflow )
• it aligns with commitEditableInstance and obtainEditable instance.

The methods throw an exception in the DefaultWorkflowImpl because there are no valid uses cases (yet) for saveDraft and editDraft for images, assets and doc types.