Refactoring

* Divide modules into low-level ('utils') and high-level ('scripts')
* Renamed some modules, added some new modules
* Each group (css/pef/liblouis/...) now has a respective utils module that
  acts as a facade for other modules. It collects all XSLT functions / XProc steps
  into a single library.xsl / library.xpl to make importing easier. Some
  XSLT libraries are just 'dummy' libraries that can be imported to make it
  clear that certain Saxon XPath extension functions are used.