MovieButton: use _movie.playChildrenOnly
GridLayoutSprite: use proper offset between rows/columns
GridLayoutSprite layout fix
fix demo
UrlDataLoader: don't wrap error events in errors
Flashbang.onFatalError takes an untyped argument

(Errors can be Error objects, ErrorEvent objects, or anything else)
Added FlashbangConfig.buildId

Show build ID on ErrorScreen if it's been supplied
DataLoader changes

- Get rid of the stateChanged signal, which is redundant (since
DataLoader is a Future)
- add a canceled signal