ENH: Add vtkMRMLCommandLineModuleNode::Completing

* Add vtkMRMLCommandLineModuleNode::Completing: state before the outputs
are loaded into the scene
* Add vtkMRMLCommandLineModuleNode::StatusModifiedEvent: event invoked when
status is changed.
* Remove unused wait_for_module(module, time_pause=5): should use the
Completing/Completed mechanism instead.