ENH: Data specified at command line are loaded into data dialog.

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