German translation (de_DE)

- Based on the English translation en_GB
- GPL-3 snippets shamelessly copied from <http://www.gnu.de/documents/gpl.de.html> as referred to by <https://www.gnu.org/licenses/translations.html>. I have no idea what the current practice is in translations of other GPLed programs. This German translation is labeled "unofficial" (see the gnu.org page again) but clearly transports the message. Maybe the English original would be better?
- msgids "read" and "write" are in German not  as fungible as the English versions. As they seem only to appear in an I/O error dialouge, I used them as part of a compositum in the dialouge text #: ../src/ui.c:29. These msgids can now be used only as the first part of a nominal compositum.