Ensure that the correct playlist is played (#5929)

When the playlist doubleclick behavior is set to add songs to the queue,
the playback of the queue also starts if no song is currently
playing. To play the queue from the current playlist, we must set the
active playlist to the current one before starting the playback.

Fixes #5714.

src/ui/mainwindow.cpp

@@ -1354,6 +1354,7 @@ void MainWindow::PlaylistDoubleClick(const QModelIndex& index) {
       app_->playlist_manager()->current()->queue()->ToggleTracks(
           dummyIndexList);
       if (app_->player()->GetState() != Engine::Playing) {
+        app_->playlist_manager()->SetActiveToCurrent();
         app_->player()->PlayAt(
             app_->playlist_manager()->current()->queue()->TakeNext(),
             Engine::Manual, true);