Docx Reader: Start list of polyglot section headers.
Docx reader: Use polyglot header list.

We're just keeping a list of header formats that different languages use
as their default styles. At the moment, we have English, German, Danish,
and French. We can continue to add to this.

This is simpler than parsing the styles file, and perhaps less
error-prone, since there seems to be some variations, even within a
language, of how a style file will define headers.
Docx reader: Add test for polyglot headers.

Only Danish at the moment.
Docx Reader: Remove header class properly in other langs

When we encounter one of the polyglot header styles, we want to remove
that from the par styles after we convert to a header. To do that, we
have to keep track of the style name, and remove it appropriately.
Merge from upstream.