qt tx dialog: always show input amounts if we know them

Previously we would only show input amounts for partial txs.
Now also show them for complete txs as well, if we know them:
we check in the wallet db for the prevtx and read the value for the output.
This is safe as the input commits to the prevout via txid (which commits to the output value).

Also show "from addresses" in more cases in a similar fashion.

electrum/address_synchronizer.py

@@ -139,6 +139,19 @@ class AddressSynchronizer(Logger):
             for n, v, is_cb in l:
                 if n == prevout_n:
                     return addr
+        tx = self.db.get_transaction(prevout_hash)
+        if tx:
+            return tx.outputs()[prevout_n].address
+        return None
+
+    def get_txin_value(self, txin: TxInput) -> Optional[int]:
+        if txin.value_sats() is not None:
+            return txin.value_sats()
+        prevout_hash = txin.prevout.txid.hex()
+        prevout_n = txin.prevout.out_idx
+        tx = self.db.get_transaction(prevout_hash)
+        if tx:
+            return tx.outputs()[prevout_n].value
         return None
 
     def get_txout_address(self, txo: TxOutput) -> Optional[str]:

electrum/gui/qt/transaction_dialog.py

@@ -551,8 +551,9 @@ class BaseTxDialog(QDialog, MessageBoxMixin):
                 if addr is None:
                     addr = ''
                 cursor.insertText(addr, text_format(addr))
-                if isinstance(txin, PartialTxInput) and txin.value_sats() is not None:
-                    cursor.insertText(format_amount(txin.value_sats()), ext)
+                txin_value = self.wallet.get_txin_value(txin)
+                if txin_value is not None:
+                    cursor.insertText(format_amount(txin_value), ext)
             cursor.insertBlock()
 
         self.outputs_header.setText(_("Outputs") + ' (%d)'%len(self.tx.outputs()))