Allow drag and drop of songs to favorite playlists

- allows adding songs from active playlist
to any favorite by drag & drop
- after 500msec hovering with the songs over
desired playlist it becomes current
- drag & drop multiple songs is supported

src/playlist/playlistlistcontainer.h

@@ -63,6 +63,7 @@ class PlaylistListContainer : public QWidget {
   // From the UI
   void NewFolderClicked();
   void ItemDoubleClicked(const QModelIndex &proxy_idx);
+  void ItemMimeDataDropped(const QModelIndex &proxy_idx, const QMimeData *q_mimedata);
 
   // From the model
   void PlaylistPathChanged(const int id, const QString &new_path);