ooxml: preserve w:sdt blocks containing w:docPartObj properties

The <w:docPartObj> property inside <w:sdt> blocks determines a block
inside the document that can be dinamically updated and enables Word
2010 to show a hover button to update it. It is used for TOCs,
bibliography, etc. LibreOffice ignored these blocks and removed them
from the document on export.

In this patch, we make the importer save the <w:docPartObj> tag and
its contents in the paragraph interop grab bag.

On export we read the paragraph grab bag and restore the sdt block
back to the document; we don't know if the paragraph must be enclosed
in a sdt block when we start it, so we used the parser marks to be
able to prepend the start of the block before the paragraph opening
tag.

The grab bag on import is managed by the SdtHelper class. Added a set
of methods for that purpose.

The ooxml model file was modified to assign token ids to the children
of w:docPartObj.

Fixed several unit tests that didn't expect the <w:sdt> tag to be
exported. Also modified testBibliography inside ooxmlexport test
suite to add checks for this patch.

TODO: in the imported documents the sdtContent block may contain
several paragraphs but the exporter code as it is can only wrap one
paragraph. As a result, if the sdt block contained several paragraphs
the second and next paragraphs will be outside the block in the
exported document.

Change-Id: I5333fc5ad91a3c50198a4f7647424a2101268c12