Use the same GUID in the file dialog for the FILESAVE_AUTOEXTENSION
case as for the FILESAVE_AUTOEXTENSION_SELECTION case. The former is
used when exporting to PDF. Exactly why using one GUID for the dialog
causes it to use the default folder as set by SetFolder(), and the
other not, I have no idea. Are the GUIDs we use arbitrary (and unique
to the OOo/LO codebase), or does either of them, or both, have some
magic meaning to Windows?

FILESAVE_AUTOEXTENSION is also used in other cases than exporting to
PDF. It remains to be seen whether this change will cause noticeable
behaviour change interpreted as a harmful regression. Or whether the
change of default folder for PDF export will do that, for that matter.

Change-Id: I135e4b2b2bc4dc70670919fb478196e27b55ac79