fdo#68099 Move pdf viewer call to GUI related code

This change fixes two bugs:
1) In it's old location at PDFFilter::implExport, the pdf viewer is called
*before* the final file is actually written. It causes a problem under
Windows, because we use ShellExecute function which fails if the file
doesn't exist. Calling a function blocks execution, so we have no chance
to write the file before that function fails. (fdo#68099)
2) Being a part of the saving/filter code means that the pdf viewer will
open even when converting to a pdf in headless mode. It's definitely not
something that we want to happen.

Change-Id: I1a6f70ec76100ac9e008869aa57ad738ced3ab6c
Reviewed-on: https://gerrit.libreoffice.org/6565
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>