Support informational updates

Support updates that aren't a software download but that allow the dev to share info with users, e.g. if they need to purchase a new version of the software or something.

This would be a rather extensive change throughout the app cast gen, main lib, and UIs to handle this properly. Right now, there is no need for this feature that I know of, and no one has ever asked for it that I know of, but I'm filing this issue as the main Sparkle lib (and WinSparkle?) supports it and someone might wonder.

Contributions are welcome.