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.

i3-input/i3-input.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <config.h>
+
 #include <err.h>
 
 #define die(...) errx(EXIT_FAILURE, __VA_ARGS__);

i3-input/main.c

@@ -8,6 +8,8 @@
  *                  to i3.
  *
  */
+#include "libi3.h"
+
 #include <stdio.h>
 #include <sys/types.h>
 #include <stdlib.h>
@@ -31,8 +33,6 @@
 
 #include "i3-input.h"
 
-#include "libi3.h"
-
 /* IPC format string. %s will be replaced with what the user entered, then
  * the command will be sent to i3 */
 static char *format;