Translate comments, fix code style, clean up code

Much of the code is commented in German and uses a code style which I don't like very much.

In many parts of the code, functions and members are also quite messy and in weird order (this is probably due to the code growing over time).

The important part is to normalize the code documentation (use English comments everywhere, use proper Doxygen format for element docs) and cleaning up the code (fix element order, remove unused elements).

Since this means that many files will be substantially changed anyway, this may as well include code style  fixes too, like replacing tabs with spaces and other things.

I'll soon post some guidelines for code style and documentation style, so other people can help with this.

To make these changes available in master (the changes in this branch should not change any semantics of the code, merely aesthetics and cosmetics), I'll merge it into master every now and then, and then add a new PR for the next changes.