MainWindow: use references instead of pointers

There is no reason why the ytdl class should be used on the heap instead
of the stack.

mainwindow.cpp

@@ -34,8 +34,8 @@ void MainWindow::on_pushButton_clicked()
 void MainWindow::actionRetrieveUrl()
 {
  this->setCursor(Qt::WaitCursor);
- YoutubeDL *ytdl = new YoutubeDL();
- QString output = ytdl->getUrl(this->ui->videoUrlEdit->text());
+ YoutubeDL ytdl;
+ QString output = ytdl.getUrl(this->ui->videoUrlEdit->text());
  OutputWindow *outputWindow = new OutputWindow();
  outputWindow->setText(output);
  outputWindow->show();
@@ -49,9 +49,9 @@ void MainWindow::actionDownload()
  {
  OutputWindow *outputWindow = new OutputWindow();
  outputWindow->show();
- YoutubeDL *ytdl = new YoutubeDL();
- outputWindow->setYtdl(ytdl->getYtdl());
- outputWindow->connect(ytdl->getYtdl(), SIGNAL(readyRead()), outputWindow, SLOT(readyRead()));
- ytdl->startDownload(this->ui->videoUrlEdit->text(), saveDirectory);
+ YoutubeDL ytdl;
+ outputWindow->setYtdl(ytdl.getYtdl());
+ outputWindow->connect(ytdl.getYtdl(), SIGNAL(readyRead()), outputWindow, SLOT(readyRead()));
+ ytdl.startDownload(this->ui->videoUrlEdit->text(), saveDirectory);
  }
 }