GerritEvent: Add notify setting

Add a notify setting so that listeners can respect the setting that was
requested in the operation that caused the event.

There are currently no listeners that use it, so for now, for the events
that are likely to need it in future, just return "ALL". These events
will be adjusted in later commits to return the correct values.

For classes that are not likely to care about this setting, inherit from
a new intermediate class that always returns "NONE".

Change-Id: I23c36b54d0d9730bfa47b1ebba7522048eee796f

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

@@ -14,6 +14,9 @@
 
 package com.google.gerrit.extensions.events;
 
+import com.google.gerrit.extensions.api.changes.NotifyHandling;
+
 /** Base interface to be extended by Events. */
 public interface GerritEvent {
+  NotifyHandling getNotify();
 }