Codechange: use \u to indicate unicode chars in strings (#8379)

With \x, we sometimes had to do the "" trick, as the length is not
predefined. With C++11 bringing \u to the specs, which has a preset
length, we no longer need the "" trick.

We set the strings to u8, to ensure all compilers use UTF-8 encoding
for the \u characters.

This was triggered by newer CLangs, which start to warn if you
use "" in the middle of a string, wondering if that was your
intention. It is a good question. And this is our answer :)