Qt: Simplify AboutDialog creation

Just create the AboutDialog on the stack -- the actual object lives on
the heap anyway, since Qt uses the pimpl idiom. Removes the need for
an explicit new and a special delete on close attribute.

Source/Core/DolphinQt2/MainWindow.cpp

@@ -553,8 +553,8 @@ void MainWindow::ShowAudioWindow()
 
 void MainWindow::ShowAboutDialog()
 {
-  AboutDialog* about = new AboutDialog(this);
-  about->show();
+  AboutDialog about{this};
+  about.exec();
 }
 
 void MainWindow::ShowHotkeyDialog()