Add a checkModified() operation to EditableWorkflow to replace the "isModified" workflow hint

Currently the BasicReviewedActionsWorkflowImpl provides a "isModified" boolean hint value, which is derived already during instantiation of the workflow itself.
By replacing this with a EditableWorkflow.checkModified() operation this derivation can be postponed until actually needed.

To help the CMS to determine if it actually makes sense to invoke this operation, the BasicReviewedActionsWorkflowImpl could return a new "checkModified" hint with as boolean value = (Workflow subject == 'draft' variant && exists('unpublished' variant))