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-nagbar/main.c

@@ -8,6 +8,8 @@
  * when the user has an error in their configuration file.
  *
  */
+#include "libi3.h"
+
 #include <stdio.h>
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -30,7 +32,6 @@
 #include <xcb/randr.h>
 #include <xcb/xcb_cursor.h>
 
-#include "libi3.h"
 #include "i3-nagbar.h"
 
 /** This is the equivalent of XC_left_ptr. I’m not sure why xcb doesn’t have a