ENH: check for data that will get deleted on scene view restore

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, add 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 0003956

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24493 3bd1e089-480b-0410-8dfb-8563597acbee