clang-format: introduce top-level config file

[ThomasAdam]

Fvwm has had a long history, with contributions from many people over the years. As a result the code formatting which has crept in is varied -- both in terms of whitespace, indentation, etc., with there being mass inconsistencies all over the code base.

Even as far back as 2006 (see commit: 1c9e17f), there was an attempt at reformatting some of FvwmButton's code, but not all of it.

Thankfully, technology exists to bring consistency to this automatically. clang-format (from llvm) is one example, but it seems to do the job well enough.

This definition cribs from FreeBSD's style, which isn't too far off the codebase as it stands at the moment.

To see what this looks like, checkout the ta/reformatted-via-clangf branch.

This was generated by limiting the code changes to fvwm's files, by running:

find fvwm libs modules -iname '*.[ch]' -exec clang-format -i --style="file:$(pwd)/.clang-format" {} \;

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Things to think about:

• PRs -- we should have a step in CI to run clang-format over the code which has changed, and flag up any inconsistencies. See: https://github.com/llvm-mirror/clang/blob/master/tools/clang-format/clang-format-diff.py -- additionally, we could use a git-commit hook as well.
• Using git to ignore these big whitespace changes when using git blame, etc. See: https://tekin.co.uk/2020/09/ignore-linting-and-formatting-commits-when-running-git-blame