Fallback playlists for filling empty shows, or gaps within shows

Fallback playlists are simpler than smart playlists. They are used to fill an empty show or a content gap within a show using a quasi-random selection of files from the storage server, of precisely the correct length to fill the show.

In this way, fallback scripts differ from random play scripts which have to be cut off when the next show starts, maybe half way through a file.