[RFC] Update German translation.

Following the steps in #767, I updated the German translation. All strings (incl. fuzzy) are now translated;  `make de.ck` reports no errors. I also changed a few obvious things when I saw them (e.g. fixing some [Leerzeichen in Komposita](https://de.wikipedia.org/wiki/Leerzeichen_in_Komposita)). I did not go through the translations line-by-line.

For vimscript-related messages, I changed the translations to use Dictionary, Number, List, etc. for the data types instead of a translation. Afaik, there is no German vimscript reference, and translating the data types might be a bit confusing and introduce ambiguity.

Most of the undo-related terms had not yet been translated, but the existing ones used "Wiederherstellung" for "undo". I don't think that really describes what undo does, but then again, I didn't want to call "undo file" "Rückgängig-Datei" or something. I used "Wiederherstellungsdatei" to be consistent, but maybe someone has a better idea for that.

Unfortunately, executing `make de` changed most of the po file (line numbers were updated), which makes this commit very big and probably hard to review :-(