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?

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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.
The only problem with the class approach is that it makes it difficult/impossible to manage extra state on custom views, which is very handy. I'll see if I can come up with another alternative.