Playlist fixes

- Fix bug resetting playlist view columns to show all when using more than one
playlist.
- Add queue to play next

src/core/mainwindow.h

@@ -168,6 +168,7 @@ signals:
   void PlaylistPlay();
   void PlaylistStopAfter();
   void PlaylistQueue();
+  void PlaylistQueuePlayNext();
   void PlaylistSkip();
   void PlaylistRemoveCurrent();
   void PlaylistEditFinished(const QModelIndex& index);
@@ -360,6 +361,7 @@ signals:
 #endif
   QAction *playlist_open_in_browser_;
   QAction *playlist_queue_;
+  QAction* playlist_queue_play_next_;
   QAction *playlist_skip_;
   QAction *playlist_add_to_another_;
   QList<QAction*> playlistitem_actions_;