Introduce clang-format + make the CI enforce clang-format clean C/C++ code#260

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hartwork wants to merge 4 commits intomasterfrom

Commits on Feb 9, 2023

.clang-format: Init with "clang-format-15 --dump-config > .clang-format"
hartwork
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.clang-format: Do not sort includes (because of unpredictable effects)
hartwork
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Mass-apply clang-format 15.0.7

clang-format
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hartwork
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Actions (clang-format): Enforce clang-format-15 clean C/C++ code
hartwork
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