- Fixes extension check.
- Looks for images relative to quickbook file instead of the current
working directory, so that it will work when run from another
directory.
- Allow user to specify the image location.
- Use `bbk_value` for synthesized attributes rather than `qbk_value`.
It's the right thing to do, but doesn't make any practical difference.
Still not happy with image path handling. I feel that they should be
relative to the quickbook file and be rewritten - in a similar manner to
`xinclude`.
[SVN r75612]
