Embedded native data (what we don't parse just carry on) and real OLE2
embedding already worked, this adds the case where the actual content is
ODF, just inside OLE2.

The DOC import/export had support for handleing ODF content inside OLE2,
so reuse that code: add new functions to SvxMSDffManager for import
purposes and reuse SvxMSExportOLEObjects for export purposes.

Change-Id: I0acf65d4bf29af896b8f1dd625e8672050aae350
Reviewed-on: https://gerrit.libreoffice.org/55088Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>