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.
Good point. I didn't think it mattered since only one notification can be shown at a time, but you're right that I should probably err on the safe side.