Add clang format to project

[xezon]

Add clang format to project.

Reference discussion

• Code formatting #409

We also need a wiki that explains in easy step how to setup (the right version of) clang format in popular IDE's and how to best use it (format on save).

And we need a wiki for how to retroactively apply our clang format to established changes, for those trying to port their changes to our code base. Otherwise they will run into huge merge conflicts which will be impossible to solve.