BUG: Consider specific file writer before generic one.

Ideally the file reader/writer should be reworked to have a more
consistent API. More specifically after the user select a file type
in the "save dialog", the corresponding "file writer id" should be
passed down the qSlicerCoreIOManager::saveScene/saveNodes methods.

This would prevent confusion for the user. Indeed, if a writer fails a
clear error message would be displated and the system wouldn't try an other
possible writer.

See issue 0003322

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22337 3bd1e089-480b-0410-8dfb-8563597acbee