Add effect that can show or hide character avatars

[Logodaedalus]

This would be good to manage having one-off characters that only appear for part of a scene, or removing characters when they leave / die / etc.

[Logodaedalus]

It's tempting to add some sort of failsafe in here that just shows all the avatars that a given fragment involves, but I think there are too many edge cases. Best to make it explicit, but design-wise authors will need to be careful to make sure if they disable character avatars that they show them at every spot they should, and vice-versa.