remove the urgency indicator when a window is closed

Long description:

If a window with the urgency hint was unmap, the workspace still showed
the indicator

To apply this patch, use:
curl http://cr.i3wm.org/patch/18/raw.patch | git am

@@ -466,6 +466,13 @@ static void handle_unmap_notify_event(xcb_unmap_notify_event_t *event) {

         goto ignore_end;

     }

+    /* remove the urgency hint of the worskpace (if set) */

+    if (con->urgent) {

+        con->urgent = false;

+        con_update_parents_urgency(con);

+        workspace_update_urgent_flag(con_get_workspace(con));

+    }

+

     tree_close(con, DONT_KILL_WINDOW, false, false);

     tree_render();

     x_push_changes(croot);