Move draw_util.c to libi3.

In order to prepare for using cairo for rendering i3 decorations,
we need to make the draw_util.c from i3bar available via libi3 such
that both i3bar and i3 can use it.

relates to #1278

i3bar/include/common.h

@@ -88,4 +88,3 @@ TAILQ_HEAD(statusline_head, status_block) statusline_head;
 #include "config.h"
 #include "libi3.h"
 #include "parse_json_header.h"
-#include "draw_util.h"