#12 Open/save other file formats

[Christian Wichmann]

Add im-/export for foreign file formats. (See also http://webpbn.com/export.cgi, http://webpbn.com/pbn_fmt.html, http://www.comp.lancs.ac.uk/~ss/nonogram/fmt2, http://www.comp.lancs.ac.uk/~ss/nonogram/xmlfmt)