Add German translation

It’s taken me a while, but I’m finally done with my German translation of the walkthrough! (Updated with the latest code changes on `1.1`.)

To review the translation (pretty please! I’m not *that* confident about it), I recommend looking at ee145b09f8a963e7bd9e0ecc4dbce1d04a4b5b22 in split diff mode. (In case it’s GCed later, I overwrote the english files with the german ones so you get a nice diff.)

5ca70fd is, strictly speaking, not related… I just did it because seeing the backup files in my `git st` was very annoying. (I wrote this mostly during train travels, on a netbook not really powerful enough for a better text editor than `emacs`.) If you don’t want it, I can remove it.
I think the idea behind the name “linked mode” is that different areas of the text update as you type, being “linked” to your current position. For example, you also enter this mode with the rename refactoring, which has nothing to do with arguments…

A phrasing that avoids the name of the mode altogether would probably be best ;-)