Removed extra add prefix handler call, since the @header decorator already registers MainMenu() as the handler.

We don't need to include the art and thumb arguments because we're already using the default names for those files.
Set NAME as the default title1 for ObjectContainer.

This is unrelated to the duplicate add prefix handler call, just nice to have.
Merge pull request #1 from jefferyto/double-handler-fix

Good explanation, that's it - yes, I am using the default names.
Merged.