Make use of `title` for `Thing` subclasses

At the moment, the `title` of a `Thing` is just the name of the class. This might be a reasonable default, but it's not particularly human-readable. It would be very nice to make it possible to set the title per-class, or even per-instance, because that would be useful for, for example, the UI.

This may already be possible, but I've not checked or documented it.