Add dedicated annotations for Views handlers

In order to better define what it means to be a views handler, we can stop using the generic @Plugin in favor of a specific @ViewsHandler.

Handlers are pretty straightforward, and actually don't need anything other than their ID.

This will also let us document the annotation keys better (even more important for non-handlers, I'll open that issue tomorrow).

Now that services are in YAML, we can individually swap out each handler type as we go. I've converted relationships, since there are so few of them.

This is mostly @msonnabaum's idea, but I happen to agree with him.

This will also prevent people from tacking on arbitrary data to the annotation.