BUG: In Slicer session, keep track of Scene file format used to save

Within a given a Slicer session, the selected Scene file format will
be remembered if the scene could be successfully saved/exported.
Note that the corresponding value won't be saved into the settings.

See 0002948

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22688 3bd1e089-480b-0410-8dfb-8563597acbee