[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/ManuallyAbandonedConversationCanceller.java

@@ -0,0 +1,19 @@
+package org.bukkit.conversations;
+
+/**
+ * The ManuallyAbandonedConversationCanceller is only used as part of a {@link ConversationAbandonedEvent} to indicate
+ * that the conversation was manually abandoned by programatically calling the abandon() method on it.
+ */
+public class ManuallyAbandonedConversationCanceller implements ConversationCanceller{
+    public void setConversation(Conversation conversation) {
+        throw new UnsupportedOperationException();
+    }
+
+    public boolean cancelBasedOnInput(ConversationContext context, String input) {
+        throw new UnsupportedOperationException();
+    }
+
+    public ConversationCanceller clone() {
+        throw new UnsupportedOperationException();
+    }
+}