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.

include/restore_layout.h

@@ -10,6 +10,8 @@
  */
 #pragma once
 
+#include <config.h>
+
 /**
  * Opens a separate connection to X11 for placeholder windows when restoring
  * layouts. This is done as a safety measure (users can xkill a placeholder