menu: use wl_list instead of array

...to make it easier to split menu.c into smaller chunks without
exposing nr_menus variable.

src/main.c

@@ -180,7 +180,7 @@ main(int argc, char *argv[])
 
 	server_finish(&server);
 
-	menu_finish();
+	menu_finish(&server);
 	theme_finish(&theme);
 	rcxml_finish();
 	font_finish();