psppire-output-view: Avoid doubling output when exporting or printing.

Outputting a group outputs its contents, but the code here was also
separately outputing the contents, which effectively doubled it.  (PSPP
doesn't currently use nested groups but these would have led to additional
copies.)

This fixes the problem by only printing items that aren't nested inside
any group.

Bug #60464.
Thanks to John Darrington for reporting this bug.