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...
The problem this solves is loading and re-saving a document
that uses an odd file structure (e.g. the main body is
not at word/document.xml). I've seen this occur with spreadsheets
but not with a docx in the wild, however it's fairly simple
to handle in case it occurs.
Decoding was getting fairly complicated, it seems cleaner
to have something handle the relationships file and traversal
and the document just be notified via callback when new
files have been found.
