Inheritance was overkill here, and now we can use inheritance for new functionality X
without having to create classes for all combinations of {X, is_testnet}.
When "importing" a psbt, we accept witness utxos even for legacy inputs
(warning shown to user in gui).
When "exporting" a psbt, we follow the spec; except when exporting as a QR code,
in which case we include witness utxos for all inputs.
This makes QR codes for psbts with legacy inputs feasible, just like they
were before, with our custom tx serialization format (with the same risk,
of burning coins as miner fees).
