Remove FTP support from the core of the Filesystem\File and Filesystem\Folder

This also removes the ClientHelper class and tests as well as the ClientHelper references from Factory. This brings us one more step closer to dropping Factory as well.

Finally, this moves FtpClient out of the Client package and into Filesystem\Clients\FtpClient. If you need FTP support, use it directly, instead of through the File / Folder classes.