BUG: Accordion widget doesn't actually accept 'true' for 'active'

The accordion widget was previously being passed true and false, whereas it either expects false or the integer of the panel to show [http://docs.jquery.com/UI/API/1.8/Accordion#option-active].

This fix sets it as either false or 0. I'm making the assumption that with the Silverstripe implementation with ToggleCompositeField, there would only ever be a single panel. If the field allows for more than one panel, then we would need to alter the `setStartClosed()` function to allow for an integer rather than just true/false.