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/draw_util.c

@@ -6,6 +6,8 @@
  * draw.c: Utility for drawing.
  *
  */
+#include "libi3.h"
+
 #include <stdlib.h>
 #include <err.h>
 #include <string.h>
@@ -13,8 +15,6 @@
 #include <xcb/xcb_aux.h>
 #include <cairo/cairo-xcb.h>
 
-#include "libi3.h"
-
 /* The default visual_type to use if none is specified when creating the surface. Must be defined globally. */
 xcb_visualtype_t *visual_type;