Steps to reproduce the issue

1. Go to Backend->Extensions->Manage->Update Sites and (un)publish an update site in the listing.
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 Update sites.

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/updatesites.php called OnUpdateSiteChangeStage. Add the following code at line 109 (in the function publish):

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

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