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.

.woodpecker/clang-tidy-checks

@@ -1,6 +1,7 @@
 # checks
 clang-analyzer-*,clang-diagnostic-*,bugprone-*,performance-*,modernize-*
 misc-*,readability-*,android-cloexec-*,cert-*
+llvm-include-order
 
 # silence
 -misc-unused-parameters