wallet.bump_fee() only allows targeting a feerate.
Prior to this commit, _BaseRBFDialog(TxEditor) allowed setting either a feerate or an abs fee.
When setting an abs fee, TxEditor.update_fee_fields() tries to adjust the feerate accordingly,
and then via side-effecting, wallet.bump_fee() will get called with the derived feerate.
This seems really buggy atm. I think it is best to disable setting abs fees, and
if we want to enable it later, targeting needs to be implemented in wallet.bump_fee() -
just like how it works in ConfirmTxDialog(TxEditor) and wallet.make_unsigned_transaction().
ce8f564fb4