Emit event when removing a mention

I've taken a look to the code and it doesn't seem to be a way to get notified when a mention is being removed.

We're already being notified when a mention is getting applied, so why don't do the same for this? I think it would be super useful if anyone needs to manage mentions.

I guess I'll be able to manage them on my own in the meantime, but it would be a **nice to have** if the `Mentionable` component already provides support for that.