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.
And `Class` is not nearly as type safe :frowning:. You'd end up getting runtime errors if you pass something that's not a subclass of `CRToastView`.