Nstack.appOpen is not cancellable

Make `Nstack.appOpen` cancellable or add a timeout parameter to set the maximum duration `appOpen()` can run for.
Because `appOpen` is not currently cancellable, code like the following will not timeout, if `appOpen()` will take more than 2000ms:
```
withTimeoutOrNull(2000) { NStack.appOpen() }
```
In this case the Coroutine will not return in 2 seconds.