CVS update: PNG codec can read and write

PNG codec read everything that I've thrown at it. It can also save images now. I've removed the many String constants from Strings and put them into three text files en.txt, de.txt and es.txt, for the three supported languages (you're welcome to help add more languages). Strings now gets those String values from the text files, loaded with a new class StringLoader. I've added package.html files to all packages where they were missing.