Ensure all *.[ch] files include config.h

Including config.h is necessary to get e.g. the _GNU_SOURCE define and
any other definitions that autoconf declares. Hence, config.h needs to
be included as the first header in each file.

This is done either via:
1. Including "common.h" (i3bar)
2. Including "libi3.h"
3. Including "all.h" (i3)
4. Including <config.h> directly

Also remove now-unused I3__FILE__, add copyright/license statement
where missing and switch include/all.h to #pragma once.

libi3/format_placeholders.c

@@ -5,12 +5,12 @@
  * © 2009 Michael Stapelberg and contributors (see also: LICENSE)
  *
  */
+#include "libi3.h"
+
 #include <stdlib.h>
 #include <stdint.h>
 #include <string.h>
 
-#include "libi3.h"
-
 #ifndef STARTS_WITH
 #define STARTS_WITH(string, needle) (strncasecmp((string), (needle), strlen((needle))) == 0)
 #endif