Make it easier to create cells manually

Alright, this might be a more controversial change. And I'm presenting it to start a discussion.

The problem is that the current design makes it really hard to use cells which are loaded from a nib. That *might* be a good thing if/since all subviews should be in `contentView`. There's no non-terrible way to do that in a nib, so it could be reasonable to disallow nibs.

If that's the case, then :-1: this PR.