added a persistent cache mechanism; used so far to cache image ids in flat view mode

git-svn-id: http://piwigo.org/svn/trunk@28432 68402e56-0260-453c-a942-63ccdbb3a9ee

admin/plugins_installed.php

@@ -77,6 +77,7 @@ if (isset($_GET['action']) and isset($_GET['plugin']))
       if ($_GET['action'] == 'activate' or $_GET['action'] == 'deactivate')
       {
         $template->delete_compiled_templates();
+        $persistent_cache->purge(true);
       }
       redirect($base_url);
     }