[workflow] finish refactoring introduced by 021035b9a7ab for 'go back' transition: * fix SubWorkflowTransition.destination() prototype (must now take an entity as argument) * fix the change state for to give the entity to the transition's destination() method * we need a new method to draw workflow since we've no entity to give and 'go back' transition usually go back to multiple states

""" CW - nevow/twisted client

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"""