sort and group includes

* 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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