Initial implementation of WB-768: Add a button to the scene file dialog that...

Initial implementation of WB-768: Add a button to the scene file dialog that shows all data files in selected scene file.   For now, a button containing the text "List Files..." has been added near the top of the Scene Dialog, just to the left of the "Open" button.  When the "List Files..." button is clicked, a dialog is displayed listing all data files for all scenes in the scene file.  If any data files are missing, the path of the file is prefixed with "MISSING:".