Add native sweet fade support, ref#40

Feature description
A sweet fade is a particular crossfade between audio tracks. The fade out offset of the current playing track and the fade in offset of the next track are calculated based on audio level. Every sweet fade between a pair of tracks is different. A sweet fade is different compared to a regular crossfade, a regular crossfade is just an overlap of tracks by a fixed amount of time. Playing a playlist with sweet fade enabled delivers a continuous mix of music.

Added value of supporting sweet fades
Sweet fades is a feature that is currently supported on the Plex iOS app, a description can be found here. The Plex iOS app has to use Apple Airplay2 in order to deliver sweet fades to the SA30. This setup has the following drawbacks:

  1. If the iPad switches off then the music stops playing;
  2. Sometimes there is a noticeable interruption/hiccup in the music playback. The exact cause is unknown, it might have to do with the quality of the wifi signal of the iPad. I never experienced these interruption/hiccups with the Music Life app and SA30. The SA30 is wired;
  3. Hires music sounds different through AirPlay2 on SA30 compared to DLNA. According to this article audio is transcoded. I think lots of detail got lost in this conversion.

Proposed solution
The Music Life app can be used to compose a playlist with sweet fades enabled and send it to the SA30. The SA30 calculates the sweet fades, assures the playlist is played flawless even if the iPad is switched off.