Refactor Save a Copy, basing it on Export functionality.
SID_SAVEACOPY takes its parameters, and sends a SID_EXPORTDOC
request with them, using SID_SAVEACOPYITEM to differentiate
Save a Copy from regular Export.

Fix storing docx to url, by preventing finalizeFilter method,
in filterbase.cxx, from writing back to the original file's
MediaDescriptor.

Change-Id: I876dbe17e43b26a43f29e797fdb157e31889ee1e
Reviewed-on: https://gerrit.libreoffice.org/3355Reviewed-by: Thorsten Behrens <tbehrens@suse.com>
Tested-by: Thorsten Behrens <tbehrens@suse.com>