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/include/workspaces.h

@@ -9,10 +9,10 @@
  */
 #pragma once
 
-#include <xcb/xproto.h>
-
 #include "common.h"
 
+#include <xcb/xproto.h>
+
 typedef struct i3_ws i3_ws;
 
 TAILQ_HEAD(ws_head, i3_ws);