If the user unchecks "Save preview picture with this document" in the
dialog field and saves the document, the appropriate flag is now
written to settings.xml by introducing the setting named
"SaveThumbnail".

Change-Id: I4916b4149a9494dd1891c0e05cb9a985ba0a9121
Reviewed-on: https://gerrit.libreoffice.org/52768Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>