If checked, we download prev (parent) txs from the network, asynchronously.
This allows calculating the fee and showing "input addresses".
We could also SPV-verify the tx, to fill in missing tx_mined_status
(block height, blockhash, timestamp, short ids), but this is not done currently.
Note that there is no clean way to do this with electrum protocol 1.4:
`blockchain.transaction.get_merkle(tx_hash, height)` requires knowledge of the block height.
Loosely based on 6112fe0e51
d83863cc52