Need some kind of proxy for alerts

[noahgibbs]

Alerts aren't drawables, and don't even return a drawable object when created. There's no easy way for Shoes3 code to even dismiss them programmatically.

Shoes-Spec test code needs a way to verify their existence -- query them, check their text, and dismiss them. The obvious way to do that is an alert() finder function that returns an alert proxy, pretty much like how it works for a button. Since Shoes doesn't have an equivalent Shoes::Type::Alert object, the proxy will likely need to only exist in test code.