sw: code simplification

XEmbeddedObject already implements XClassifiedObject which contains a
getClassID method.

Change-Id: Ifbeed343493208fff3704b21ee484fd92c570974
sw: write other types of embeddings properly to docx

Improves patch 1428ec6f4e2bfe0d8654a9ccc713e274e08c6423

When embedding an object into a docx, several things interact:

* The properties in the <o:OLEObject> tag in document.xml
* The ContentType for the file defined in [Content_Types].xml
* The Type of the Relationship defined in document.xml.rels

You need the right combination of those three elements for Word to
properly recognize the embeddings in exported documents.

To know which values must be written, I store some interoperability
information in the import phase in the document grab bag. The
relevant information is the value of the ProgID attribute in the
<o:OLEObject> tag.

I have defined three cases depending on the value of ProgID, but more
could be needed in the future:
* Embedded xlsx sheet.
* Embedded pptx presentation.
* Generic OLE, this should work with embedded odt/ods in combination
  with the ProgID attribute stored in the import phase.

Change-Id: I26336cb3fe47bd33e1cef11dd1c7edcf390f2e56