Added Input.PageUp and Input.PageDown methods to JSON-RPC

cherry-picked from https://github.com/plexinc/plex-home-theater-public/commit/270384ad6
Added Input.NextLetter and Input.PreviousLetter methods to JSON-RPC

cherry-picked from https://github.com/plexinc/plex-home-theater-public/commit/270384ad6