Added new readdir() method to all storage classes and handlers (only working implementation in local.php)

to make it more generic.

allows to easily convert strings to HTML before displaying them on the web page
to reduce the risk of xss vulnerabilities.

This reverts commit 709dbd82.

when checking if the content of a folder has been updated, exlicitly state that we are checking a folder

solves some issues with external storages not updating correctly