Desktop and i18n work

Created by: mwilck

This started out with me realizing that we had no translations in our .desktop file. I thought that's not ideal for a desktop program. I tried to figure out how to generate them and stumbled upon the recent move from intltool to gettext, and figured that the .po files haven't been updated for quite a while. It was a pretty rough ride because I'm not familiar with either waf or gettext.

Eventually I have come up with a way to rebuild the .po files from the current source, and optionally commit them in in git. (That's the big "update translations" commit. I've reviewed the de.po file and the results seemed reasonable to me). I also added some missing German translations.

Please have a look.