BUG: Fix import of hierarchy node with existing ID

Note that it does not fix the import of annotation hierarchies.
For some reasons (see vtkSlicerAnnotationModuleLogic::OnSceneNodeAdded),
the hierarchies are automatically loaded into the wrong hierarchy node
parent.
Issue #2849

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21561 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: don't add hierarchy nodes if the scene is importing/restoring. Issue #2849

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21562 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Data specified at command line are loaded into data dialog.

Looks at all unparsed arguments, and determines first which of
them are file paths.

If there is only one argument, and that argument is a MRML scene,
the load-data-dialog is bypassed and the scene is loaded directly.

These arguments can be individual files, or entire directories.
If the path is recognized as a directory, its contents are
traversed recursively and added to the data dialog.

See http://na-mic.org/Mantis/view.php?id=2363

Part of #2363