Would be nice if it was possible if an IssueEventListener had a method that is called back when issue's watcher have been added/removed.
This can be a single event like WatchersModified containing the issue.

We need this functionality to implement an extension that would populate issue's watchers into sub-tasks' watchers - seems doesn't supported by Jira itself.
If there is another way of doing it, then probably the event may not be needed.