-Added new intents to append streams to current player.

-Added long clicks for popup and background player buttons on details fragment for append intents.
-Removed restrictions for preventing UI to show up when player is buffering.
-Fixed icons for all repeat modes.
-Added Progress bar to background activity when player is in not ready state.
-Fixed Track Selection when switching between video and audio only on video players.
-Fixed video player to enable tunnelling only after sdk > 21.
-Fixed activity exception from restarting after service is shutdown on earlier sdk versions.

app/src/main/java/org/schabi/newpipe/util/NavigationHelper.java

@@ -57,6 +57,14 @@ public class NavigationHelper {
                 .putExtra(VideoPlayer.PLAY_QUEUE, playQueue);
     }
 
+    public static Intent getPlayerIntent(final Context context,
+                                         final Class targetClazz,
+                                         final PlayQueue playQueue,
+                                         final boolean isAppending) {
+        return getPlayerIntent(context, targetClazz, playQueue)
+                .putExtra(BasePlayer.APPEND_ONLY, isAppending);
+    }
+
     public static Intent getPlayerIntent(final Context context,
                                          final Class targetClazz,
                                          final PlayQueue playQueue,