mvmOS 0.28.2-beta: App-to-app integrations work again

Apps that hand work to another app — Tasks and Shopping List writing a reward into Budget — stopped doing so when app isolation arrived in 0.28.0-beta. They work again. A call from one app to another now runs confined to the app being called, so it can open its own database instead of being judged by the confinement of whoever called it. Neither side can reach outside its own folder. The App APIs switch in Apps Hub is still the only thing that decides whether a call is allowed at all, and it is off by default — turning it on or off now takes effect immediately instead of at the next restart.