[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/Conversable.java

@@ -33,6 +33,13 @@ public interface Conversable {
      */
     public void abandonConversation(Conversation conversation);
 
+    /**
+     * Abandons an active conversation.
+     * @param conversation The conversation to abandon
+     * @param details Details about why the conversation was abandoned
+     */
+    public void abandonConversation(Conversation conversation, ConversationAbandonedEvent details);
+
     /**
      * Sends this sender a message raw
      *