The SVG output produced by external tools needs some postprocessing.
This is implemented by this new XSL stylesheet.

Issues are:

- SVG produced by Ditaa does not add a viewBox attribute to the svg
  element, needed to make the image scalable.

- SVG produced by Graphviz uses a stroke="transparent" attribute,
  which is not valid SVG.  It appears to mostly work, but FOP
  complains.

Other tweaks can be added over time.

This reverts 7dc78d8ef3e62f7e06d7767c63dcede048377b9a and
29046c44f36099f4c979b1491fcf27db2f9184f9, which applied these fixes
manually.