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.

i3bar/src/mode.c

@@ -7,6 +7,8 @@
  * mode.c: Handle mode event and show current binding mode in the bar
  *
  */
+#include "common.h"
+
 #include <string.h>
 #include <stdlib.h>
 #include <stdio.h>
@@ -14,8 +16,6 @@
 #include <yajl/yajl_parse.h>
 #include <yajl/yajl_version.h>
 
-#include "common.h"
-
 /* A datatype to pass through the callbacks to save the state */
 struct mode_json_params {
     char *json;