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...
These are implied values as they are the defaults in the XSD
for optional attributes/elements. However I've noticed that
I keep running into issues where Mac Excel/Word fail to open
a file unless an optional default is provided.
I'll just add them all to hopefully increase compatibility and
save painful file format debugging sessions later.
