Cleanup: Implicitly free QApplication

Instead of deleting the Application explicitly in exit_ui(),
use a unique_ptr to have the instance automatically freed.
This is the pattern given in the Qt-docs:
https://doc.qt.io/qt-5/qapplication.html

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

subsurface-helper.cpp

@@ -56,7 +56,6 @@ void exit_ui()
 #ifndef SUBSURFACE_MOBILE
 	delete MainWindow::instance();
 #endif // SUBSURFACE_MOBILE
-	delete qApp;
 	free((void *)existing_filename);
 }