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.

autoreload.c

@@ -23,8 +23,8 @@
 #include <errno.h>
 #include <stdlib.h>
 #include <string.h>
-#include <unistd.h>
 #include <sys/inotify.h>
+#include <unistd.h>
 
 static union {
 	char d[4096]; /* aligned buffer */