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/main.c

@@ -5,6 +5,8 @@
  * © 2010 Axel Wagner and contributors (see also: LICENSE)
  *
  */
+#include "common.h"
+
 #include <stdio.h>
 #include <i3/ipc.h>
 #include <string.h>
@@ -15,8 +17,6 @@
 #include <getopt.h>
 #include <glob.h>
 
-#include "common.h"
-
 /*
  * Having verboselog(), errorlog() and debuglog() is necessary when using libi3.
  *