Minor project structure cleanup

- DerStern.xml contained includes for both externalBibliographyTest.xml and quotes.xml.
  Renamed the first to Bibliography.xml and removed the include for the second, whose
  function didn't seem clear (the file is still there if you want to bring it back).
- Created an Oxygen project (DerStern.xpr). This doesn't do much, it's just an easy way
  to open and manage all the relevant XML and XSL files.
Added bibliography external reference support

<bibl> items can now contain a <ptr> reference to a URL
which is loaded in the right-hand pane on click.

Fixes #3