Custom order type tweaks

A few tweaks for the custom order types API.

Mostly semantic (i.e. displaying the order type's name instead of a hardcoded **Order** string).

It does include a couple of important logic changes though, like deleting order items and item meta data when deleting an instance of a custom order type and updating order counts when trashing/untrashing an instance of a custom order type that requests being included in those (SHA: 47e8ca0d93783152cbaa56e5a55e870066b82f1b). SHA: 3b917737ebc23289a4dd also ensures that meta boxes receive an instance of the custom order type rather than forcing it to `WC_Order`.

I've made the PR to the `2.2` branch so they go out with the next minor version, but let me know if you'd prefer it to go to `master` and I'll make another PR. The only issue then is that store managers will need WC 2.3 to use Subscriptions 2.0.