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).
There are now two internal strategies to bump the fee of a txn.
bump fee method 1: keep all inputs, keep all not is_mine outputs,
allow adding new inputs
bump fee method 2: keep all inputs, no new inputs are added,
allow decreasing and removing outputs (change is decreased first)
Method 2 is less "safe" as it might end up decreasing e.g. a payment to a merchant;
but e.g. if the user has sent "Max" previously, this is the only way to RBF.
We try method 1 first, and fail-over to method 2.
Previous versions always used method 2.
fixes#3652
