This adds comment support for sheets. Excel requires a VML drawing with
the comment box shape for each comment to display the comment.
LibreOffice displays comments fine with or without the shape, and
creates the shape for its own comments. For the sake of compatibility,
we create comment shapes as well.
I know of no other use for the legacy VML support other than comment
boxes...
This seems like the better choice, a lot of the logic is shared
between the document types, and it allows generating filenames in
a single place.
The only downside is that you must pass in the document type as
some content types have different typical names depending on the
document type (e.g. an 'office document' is the main document.xml,
workbook.xml and presentation.xml
