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/lib/playlist.js

@@ -0,0 +1,85 @@
+module.exports = {
+  hasNext,
+  getNextIndex,
+  hasPrevious,
+  getPreviousIndex,
+  getCurrentLocalURL
+}
+
+const TorrentSummary = require('./torrent-summary')
+const TorrentPlayer = require('./torrent-player')
+
+var cache = {
+  infoHash: null,
+  previousIndex: null,
+  currentIndex: null,
+  nextIndex: null
+}
+
+function hasNext (state) {
+  updateCache(state)
+  return cache.nextIndex !== null
+}
+
+function getNextIndex (state) {
+  updateCache(state)
+  return cache.nextIndex
+}
+
+function hasPrevious (state) {
+  updateCache(state)
+  return cache.previousIndex !== null
+}
+
+function getPreviousIndex (state) {
+  updateCache(state)
+  return cache.previousIndex
+}
+
+function getCurrentLocalURL (state) {
+  return state.server.localURL + '/' + state.playing.fileIndex
+}
+
+function updateCache (state) {
+  var infoHash = state.playing.infoHash
+  var fileIndex = state.playing.fileIndex
+
+  if (infoHash === cache.infoHash) {
+    switch (fileIndex) {
+      case cache.currentIndex:
+        return
+      case cache.nextIndex:
+        cache.previousIndex = cache.currentIndex
+        cache.currentIndex = fileIndex
+        cache.nextIndex = findNextIndex(state)
+        return
+      case cache.previousIndex:
+        cache.previousIndex = findPreviousIndex(state)
+        cache.nextIndex = cache.currentIndex
+        cache.currentIndex = fileIndex
+        return
+    }
+  } else {
+    cache.infoHash = infoHash
+  }
+
+  cache.previousIndex = findPreviousIndex(state)
+  cache.currentIndex = fileIndex
+  cache.nextIndex = findNextIndex(state)
+}
+
+function findPreviousIndex (state) {
+  var files = TorrentSummary.getByKey(state, state.playing.infoHash).files
+  for (var i = state.playing.fileIndex - 1; i >= 0; i--) {
+    if (TorrentPlayer.isPlayable(files[i])) return i
+  }
+  return null
+}
+
+function findNextIndex (state) {
+  var files = TorrentSummary.getByKey(state, state.playing.infoHash).files
+  for (var i = state.playing.fileIndex + 1; i < files.length; i++) {
+    if (TorrentPlayer.isPlayable(files[i])) return i
+  }
+  return null
+}