This adds support for extracting a cell's formatted value according to
the number format applied to the cell. To do this we need to implement
a parser for Excel style format strings and support formatting numbers
according to that style.
This also enhances the General formatting to be much closer to what
Excel normally does. There are likely still a few corner cases where
Excel and gooxml differ, but hopefully not too many.
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...
