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/main/shortcuts.js

@@ -12,9 +12,19 @@ function enable () {
     'MediaPlayPause',
     () => windows.main.dispatch('playPause')
   )
+  electron.globalShortcut.register(
+    'MediaNextTrack',
+    () => windows.main.dispatch('nextTrack')
+  )
+  electron.globalShortcut.register(
+    'MediaPreviousTrack',
+    () => windows.main.dispatch('previousTrack')
+  )
 }
 
 function disable () {
   // Return the media key to the OS, so other apps can use it.
   electron.globalShortcut.unregister('MediaPlayPause')
+  electron.globalShortcut.unregister('MediaNextTrack')
+  electron.globalShortcut.unregister('MediaPreviousTrack')
 }