* inform user about Twig cache failure on installation, instead of http 500 error

* when dir system/cache is not writable by the webserver, then show some
nice notice to the user about it instead of http 500 error
* remember client version and usage stats checkbox in session on install
* fixed some small warning introducted yesterday about
highscores_ids_hidden
* updated TODO

system/init.php

@@ -149,7 +149,7 @@ $config['data_path'] = $tmp;
 unset($tmp);
 
 // new config values for compability
-if($config['highscores_ids_hidden'] == null) {
+if(!isset($config['highscores_ids_hidden'])) {
 	$config['highscores_ids_hidden'] = array();
 }