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.

libi3/get_mod_mask.c

@@ -5,13 +5,13 @@
  * © 2009 Michael Stapelberg and contributors (see also: LICENSE)
  *
  */
+#include "libi3.h"
+
 #include <stdint.h>
 #include <stdlib.h>
 #include <xcb/xcb.h>
 #include <xcb/xcb_keysyms.h>
 
-#include "libi3.h"
-
 /*
  * All-in-one function which returns the modifier mask (XCB_MOD_MASK_*) for the
  * given keysymbol, for example for XCB_NUM_LOCK (usually configured to mod2).