Revert "Unnecessary use of a pointer for MainWindow."

This reverts commit f33695eb2f.

src/main.cpp

@@ -83,8 +83,8 @@ int Q_DECL_IMPORT main(int argc, char* argv[])
     if (!loadBackend())
         return 0;
 
-    MainWindow mainWindow;
-    mainWindow.show();
+    MainWindow* mainWindow = new MainWindow();
+    mainWindow->show();
 
     return app.exec();
 }