[Bleeding] Added ConversationAbandonedEvent and supporting infrastructure. Whenever a conversation exits, the ConversationAbandonedEvent is triggered with details about how the conversation ended and what, if anything caused it to end. Fixes BUKKIT-986

By: rmichela <deltahat@gmail.com>

paper-api/src/main/java/org/bukkit/conversations/InactivityConversationCanceller.java

@@ -48,7 +48,7 @@ public class InactivityConversationCanceller implements ConversationCanceller {
                     startTimer();
                 } else if (conversation.getState() ==  Conversation.ConversationState.STARTED) {
                     cancelling(conversation);
-                    conversation.abandon();
+                    conversation.abandon(new ConversationAbandonedEvent(conversation, InactivityConversationCanceller.this));
                 }
             }
         }, timeoutSeconds * 20);