Add .clang-format file and format code
The original format is more or less preserved, except that ContinuationIndentWidth has been changed to 4 instead of 8, and we break a little less often at the first argument of functions (and never at the first parameter of function definitions). We also try to be a little more compact overall.
The space after the negation operator '!', which I myself had introduced in the past, is dropped (I've changed my mind, it was definitely a bad idea, sorry).
Related: orage#32 (closed), xfce/xfce4-dev-tools!101 (merged)