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/config.c

@@ -7,6 +7,8 @@
  * config.c: Parses the configuration (received from i3).
  *
  */
+#include "common.h"
+
 #include <string.h>
 #include <stdlib.h>
 #include <stdio.h>
@@ -17,8 +19,6 @@
 
 #include <X11/Xlib.h>
 
-#include "common.h"
-
 static char *cur_key;
 static bool parsing_bindings;
 static bool parsing_tray_outputs;