Fallback: Trigger playout from M3U playlists and station fallback folder

• POC
• Liquidsoap should choose Playlist depending on station or schedule fallback mode (see )
• Liquidsoap to play out station fallback from folder as the last chance to save the day
• Entries should be of type "file" only
• Liquidsoap silence detector should trigger a M3U playlist
• Configuration options and documentation

Note, there might be the need to have a global "stream" or "live" Routing Option for fallback scenarios (in contrast to fallback playlists). Such sources are played in any case (Routing options are TBD).