Add confirmation before clearing playlists with more than 500 songs

src/core/mainwindow.h

@@ -155,6 +155,7 @@ class MainWindow : public QMainWindow, public PlatformInterface {
   void PlaylistSkip();
   void PlaylistRemoveCurrent();
   void PlaylistEditFinished(const QModelIndex& index);
+  void PlaylistClearCurrent();
   void RescanSongs();
   void EditTracks();
   void EditTagDialogAccepted();