[Conversation] rename Conversation::getCurrentState() to getCurrentPage()
[Conversation] rename Conversation::getPreviousState() to getPreviousPage()
[Conversation] rename the argument of Conversation::transition()
[Pageflow] add Page
[Pageflow] add Pageflow::getCurrentPage() and getPreviousPage()
[Conversation] change Conversation::getCurrentPage() and getPreviousPage() to return a Page object
[EventListener] change the message of AccessDeniedHttpException in ConversationListener
[Conversation] rename ConversationContext::getCurrentState() to getCurrentPage()
[Conversation] rename ConversationContext::getPreviousState() to getPreviousPage()
[Controller] rename ReflectionConversationalController::addAcceptableState() to addAcceptablePage()
[Controller] rename ReflectionConversationalController::getAcceptableStates() to getAcceptablePages()
[Controller] rename ReflectionConversationalController::$acceptableStates to $acceptablePages
change some properties and local variables to use the term "page" instead of "state"