fix some html problems:

- within paragraphs use span instead of div
- phrasing environments are now started automatically
- you can now nest ul lists
paint a checker board pattern behind layout to facilitat pixel counting
make it possible to change the value of already existing rules
Make documentation compatible to Doxygen 1.8.5
Update documentation for Debian
Merge pull request #25 from vog/improve-docs

Improve documentation
add a font with arabic for future tests
make sure the XML parser replaces &#xxx; with proper unicode
fix an inconsistency in the saveload routines, one used tag-name descr, others used file
Merge branch 'master' of https://github.com/roever/STLL
fix a problem with the linebreak detection:
on language borders it always wanted to break
implement loading/saving of image and rectangle commands
allow span as a html tag selector
add test so that we have now a line coverage of 99.6% for layouter.cpp

The one missing line is more or less an error case