qt tx dialog: add checkbox "Download input data"

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 https://github.com/Electron-Cash/Electron-Cash/commit/6112fe0e51e48e9ceaaecf47a014e6f4a7b41703

electrum/wallet.py

@@ -277,6 +277,7 @@ class TxWalletDetails(NamedTuple):
     mempool_depth_bytes: Optional[int]
     can_remove: bool  # whether user should be allowed to delete tx
     is_lightning_funding_tx: bool
+    is_related_to_wallet: bool
 
 
 class Abstract_Wallet(ABC, Logger, EventListener):
@@ -862,6 +863,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
             mempool_depth_bytes=exp_n,
             can_remove=can_remove,
             is_lightning_funding_tx=is_lightning_funding_tx,
+            is_related_to_wallet=is_relevant,
         )
 
     def get_tx_parents(self, txid) -> Dict: