* includes are sorted alphabetically
* their grouping and layout is the following:
- nsxiv.h will be the first include
- followed by any internal headers (e.g "commands.h" "config.h")
- followed by system headers (<stdlib.h> etc)
- followed by third party headers (X.h libwebp etc)
* also add `llvm-include-order` check to clang-tidy so that it can catch
unsorted includes during CI.
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