Add playlists feature (#871)

* Open multi-file torrents as playlists

* Add `repeat` and `shuffle` options to the player

* Autoplay first file in torrent

* replaces `pickFileToPlay` feature
* when reopening player, restores the most recently viewed file

* Add playlist navigation buttons to Windows thumbar

* Remove `repeat` and `shuffle` options

This reverts commit 9284122461.

* Play files in order they appear in torrent

* Clean up playlists code

src/renderer/main.js

@@ -189,6 +189,8 @@ const dispatchHandlers = {
   // Playback
   'playFile': (infoHash, index) => controllers.playback.playFile(infoHash, index),
   'playPause': () => controllers.playback.playPause(),
+  'nextTrack': () => controllers.playback.nextTrack(),
+  'previousTrack': () => controllers.playback.previousTrack(),
   'skip': (time) => controllers.playback.skip(time),
   'skipTo': (time) => controllers.playback.skipTo(time),
   'changePlaybackRate': (dir) => controllers.playback.changePlaybackRate(dir),