Add XSL stylesheet to fix up SVG files

Add XSL stylesheet to fix up SVG files

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.