Implement events of PublicFTP

[renatomassaro]

The PublicFTP feature is completed, but it's lacking the corresponding events (PFTPFIleAdded, PFTPFileRemoved, PFTPServerEnabled, PFTPServerDisabled).

(I mean, we need PublicFTP working for the Storyline and DownloadCenter, but player-specific mechanics/strategies involving the PublicFTP feature can wait).

For celerity and simplification, the implementation of PublicFTP events has been postponed until the need arises.