[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/ConversationAbandonedEvent.java

@@ -0,0 +1,48 @@
+package org.bukkit.conversations;
+
+import java.util.EventObject;
+
+/**
+ * ConversationAbandonedEvent contains information about an abandoned conversation.
+ */
+public class ConversationAbandonedEvent extends EventObject {
+
+    private ConversationContext context;
+    private ConversationCanceller canceller;
+
+    public ConversationAbandonedEvent(Conversation conversation) {
+        this(conversation, null);
+    }
+
+    public ConversationAbandonedEvent(Conversation conversation, ConversationCanceller canceller) {
+        super(conversation);
+        this.context = conversation.getContext();
+        this.canceller = canceller;
+    }
+
+    /**
+     * Gets the object that caused the conversation to be abandoned.
+     * @return The object that abandoned the conversation.
+     */
+    public ConversationCanceller getCanceller() {
+        return canceller;
+    }
+
+    /**
+     * Gets the abandoned conversation's conversation context.
+     * @return The abandoned conversation's conversation context.
+     */
+    public ConversationContext getContext() {
+        return context;
+    }
+
+    /**
+     * Indicates how the conversation was abandoned - naturally as part of the prompt chain or prematurely via a
+     * {@link ConversationCanceller}.
+     * @return True if the conversation is abandoned gracefully by a {@link Prompt} returning null
+     * or the next prompt. False of the conversations is abandoned prematurely by a ConversationCanceller.
+     */
+    public boolean gracefulExit() {
+        return canceller == null;
+    }
+}