- risk_of_burning_coins_as_fees is turned into a private (helper) method, only called by check_sighash.
UIs should only care about check_sighash.
- check_sighash returns instance of new class "TxSighashDanger" instead of tuple
- made warning levels more fine-grained (FEE_WARNING_SKIPCONFIRM vs FEE_WARNING_NEEDCONFIRM)
- this became more complicated than I had hoped for but I think it is worth it to ~merge
check_sighash and risk_of_burning_coins_as_fees into one.
related https://github.com/spesmilo/electrum/pull/8699