New event listeners and new event types

This is a first step towards moving ChangeHooks into a plugin.  The
listeners[1] are now invoked from all places where corresponding
hooks are invoked.  Invocation of hooks is not removed in this change.
This will happen when we make sure that they are invoked from a new
event listener.

[1] All listeners under:
gerrit-extension-api/src/main/java/com/google/gerrit/extensions/events/

Change-Id: I6e5356edb2c326d2d7a0133b84509ad6071140d0

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/events/CommentAddedListener.java

@@ -27,6 +27,7 @@ public interface CommentAddedListener {
     AccountInfo getAuthor();
     String getComment();
     Map<String, ApprovalInfo> getApprovals();
+    Map<String, ApprovalInfo> getOldApprovals();
   }
 
   void onCommentAdded(Event event);