-Added play queue dropdown to channel info items.

-Added play queue dropdown to playlist info items.
-Added Channel Play Queue.
-Renamed External Play Queue to Playlist Play Queue.
-Modified Playlist Play Queue to allow loading from initial state.

app/src/main/java/org/schabi/newpipe/player/playback/MediaSourceManager.java

@@ -157,7 +157,7 @@ public class MediaSourceManager {
     }
 
     private void onPlayQueueChanged(final PlayQueueEvent event) {
-        if (playQueue.isEmpty()) {
+        if (playQueue.isEmpty() && playQueue.isComplete()) {
             playbackListener.shutdown();
             return;
         }