gui: Fix File|Export in output viewer.

gui: Fix File|Export in output viewer.

Commit f51ecb480 "Greatly simplify PSPP configuration" changed how PSPP
output drivers are configured but failed to completely update the
implementation of File|Export to match.  This commit fixes the problem.

A few problems, perhaps not connected, linger:

    * The CSV driver appears to always output to "pspp.csv" when invoked
      from the GUI, regardless of the requested file name.

    * The ODT driver appears to be broken.

Reported by John Darrington <john@darrington.wattle.id.au>.