Add php event for altering data when changing a post's poster

I ran into this issue, and after talking with a fellow developer, this appears to affect different extensions. Let's say you have a thanks posts extension. One feature of this extension is that it shows how many thanks you have received for each post. If this data is stored in it's own table (and not one of the phpBB default tables), there is no event that will allow you to update this data when you change a post's poster using the chgposter function. I attempted to fix this by using the core.mcp_post_additional_options event and using an if ($event['action'] == 'chgposter') statement, but no luck. Looking further into it. there is a change_poster function at the end of the file that we can add an event to to fix this issue. I have an event created locally and can send a pull request.

TL/DR: We need an event for when the chgposter feature is used.