Improve sign_psbt UX for multiple outputs

[bigspider]

#270 only improved the UX for transactions with a single external output, falling back to the older UX flow if more are present; notably, for multiple outputs the flow is forced from left-to-right, and it's impossible to inspect a previous output without going rejecting and starting over.

We might improve the UX up to a fixed number of outputs, falling back to the streaming UX flows only above this number.

Alternatively, a bidirectional streaming flow could be added to the SDK, allowing to improve the UX for an arbitrary number of outputs.