Allow thirdparty modules suggest items in the autocomplete.

Hi!

In my current project I needed a way to mention custom groups of users via a custom keyword so I added a new event to the module so thirdparty modules can add their own suggestions in the autocomplete.

This patch requires #2940516: Add an event to allow thirdparty modules do something when a user is mentioned to work.

Patch in my next comment.

Thanks!

- David.