The go command on Windows doesn't like long path name. To
work around this, this is the first in a series of changes to
shorten the schema disk path structure in a few places.
Fixes#89
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...
