AppShellDataLoader<T> typedef
Loads everything an app needs before its first frame.
Called exactly once, from the shell's initState. The two apps differ here
on purpose: OmniAccelerANT reads five JSON assets (its WebRTC settings are
the fifth), jotrockenmitlocken goes through its own SettingsLoader. The
shell only cares that a future eventually produces a T.
Implementation
typedef AppShellDataLoader<T> = Future<T> Function();