GDialog: Remove self from parent when the nested event loop returns

This ensures that we close (and don't leak) the dialog during the
typical usage pattern.

Libraries/LibGUI/GDialog.cpp

@@ -29,6 +29,7 @@ int GDialog::exec()
     auto result = m_event_loop->exec();
     m_event_loop = nullptr;
     dbgprintf("%s: event loop returned with result %d\n", class_name(), result);
+    remove_from_parent();
     return result;
 }