ENH: check for data that will get deleted on scene view restore
requested to merge github/fork/naucoin/3956-warn-when-sceneview-restore-will-delete-data-squashed into master
Added a flag to the scene view node restore call that will cancel if any nodes come up as being in the main Slicer scene but not in the scene view scene about to be restored. Added a method to add nodes to the scene view scene that are in the main Slicer scene but not in the scene view. Updated the GUI for restoring scenes via the tool bar and the Scene Views module to pop up message boxes to give the user options to discard the data, save it to the scene view, or cancel the restore. Updated the test to include a failure case when removing nodes is not allowed. Unify scene view node to use SafeDownCast instead of a mix of dynamic_cast and SafeDownCast.
Issue #3956