Steps to reproduce the issue

1. Go to Backend->Extensions->Manage->Manage and (un)publish an Extension item e.g. "Articles - Archived".
2. Try to catch the event using OnContentChangeState in a Plugin (why this event? 'cause every other changestate works through this event - so it's expected to use this one ;-) ).

Expected result

The plugin function should be triggered.

Actual result

No result because the trigger does not exists for (un)publishing extensions.

System information (as much as possible)

PHP Version 7.0.9
Web Server Apache 2.4.23 (Win32)
Joomla! version 3.6.5
User Agent Mozilla/5.0 (Windows NT 6.3; WOW64; rv:50.0) Gecko/20100101 Firefox/50.0

Additional comments

Suggestion to add a trigger event to administrator/components/com_installer/models/manage.php called OnExtensionChangeStage. Add the following code at line 140:

$context = $this->option . '.' . $this->name;
JEventDispatcher::getInstance()->trigger('onExtensionChangeState', array($context, $eid, $value));

The event name onContentChangeState is not logical because this event must be started from within a System plugin (if I am right).