fdo#83751-FILESAVE:Custom Properties dropped while exporting to .pptx

Change-Id: Ic4b8acfc3cbd8c86b765a950d61742c229f3fb6d
Reviewed-on: https://gerrit.libreoffice.org/11390
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>