Add option to set a custom view for notifications.

This is just a preview PR to get some preliminary feedback before I submit this upstream. @djsell, could you take a look?

---

This changeset enables the creation of custom layouts for notifications. I had to make a few things public that weren't before, namely the option defaults system and some parts of `CRToastView`, but hopefully the extra functionality is worth this cost.
I've implemented the class-based approach in 49c5f11 and tightened validation for it in 139214d.