File cache directory can't be changed

As an developer/administrator it isn't possible to utilise the phpBB file cache but have it cache to a different directory, the cache destination is set in the constructor but as this called by the `phpbb_cache_factory` without any parameters (and you can't pass any yourself), the default case $this->cache_dir = !is_null($cache_dir) ? $cache_dir : $phpbb_root_path . 'cache/'; is used.
My proposal is to introduce a new constant PHPBB_ACM_FILE_DIR (which defaults to $phpbb_root_path . 'cache/';) that can be used to change the cache destination. As the template cache always caches to this directory it also needs to be able to read this constant, therefore it won't be possible to introduce it in `includes/cache/driver/file.php` as that file isn't included when a different cache method is used.