Fallback: Scheduled fallbacks for show & schedule

• Entries of type "stream" or "live" - such playlists should only be played when there's a recording available. In that case the recording is broadcasted (Note: this requires certain tank features to be implemented; use a stub implementation meanwhile). UPDATE: Recording not possible, see comment below)
• FallbackManager should not resolve fallback playlists in ordinary playout
• Liquidsoap silence detector should trigger a M3U playlist
• Configuration options and documentation
• Smooth fade-out for scheduled fallbacks
• Improve fade-out logic to respect fader-position