qt: kind of fix bip70 notify_merchant logic by passing around PI

```
229.18 | E | gui.qt.main_window.[test_segwit_2] | on_error
Traceback (most recent call last):
  File "...\electrum\gui\qt\util.py", line 917, in run
    result = task.task()
  File "...\electrum\gui\qt\send_tab.py", line 681, in broadcast_thread
    if self.payto_e.payment_identifier.has_expired():
AttributeError: 'NoneType' object has no attribute 'has_expired'
```

In SendTab.broadcast_transaction.broadcast_thread, self.payto_e.payment_identifier was referenced -
but do_clear() has already cleared it by then.
E.g. consider SendTab.pay_onchain_dialog: it calls save_pending_invoice(), which calls do_clear(),
and later (in sign_done), it calls window.broadcast_or_show, which will call SendTab.broadcast_transaction().

As there might be multiple independent transaction dialogs open simultaneously, the single shared state
send_tab.payto_e.payment_identifier approach was problematic -- I think it is conceptually nicer to
pass around the payment_identifiers as needed, as done with this change.

However, this change is not a full proper fix, as it still somewhat relies on
send_tab.payto_e.payment_identifier (e.g. in pay_onchain_dialog). Hence, e.g. when using
the invoice_list context menu "Pay..." item, as payto_e.payment_identifier is not set,
payment_identifier will be None in broadcast_transaction.

but at least we handle PI being None gracefully -- before this change, broadcast_transaction
expected PI to be set, and it was never set to the correct thing (as do_clear() already ran by then):
depending on timing it was either None or a new empty PI. In the former case, producing the above
traceback and hard failing (not only for bip70 stuff!), and in the latter, silently ignoring the logic bug.

electrum/gui/qt/transaction_dialog.py

@@ -70,6 +70,7 @@ from .my_treeview import create_toolbar_with_menu
 if TYPE_CHECKING:
     from .main_window import ElectrumWindow
     from electrum.wallet import Abstract_Wallet
+    from electrum.payment_identifier import PaymentIdentifier
 
 
 _logger = get_logger(__name__)
@@ -378,9 +379,16 @@ def show_transaction(
     parent: 'ElectrumWindow',
     prompt_if_unsaved: bool = False,
     external_keypairs=None,
+    payment_identifier: 'PaymentIdentifier' = None,
 ):
     try:
-        d = TxDialog(tx, parent=parent, prompt_if_unsaved=prompt_if_unsaved, external_keypairs=external_keypairs)
+        d = TxDialog(
+            tx,
+            parent=parent,
+            prompt_if_unsaved=prompt_if_unsaved,
+            external_keypairs=external_keypairs,
+            payment_identifier=payment_identifier,
+        )
     except SerializationError as e:
         _logger.exception('unable to deserialize the transaction')
         parent.show_critical(_("Electrum was unable to deserialize the transaction:") + "\n" + str(e))
@@ -392,7 +400,15 @@ class TxDialog(QDialog, MessageBoxMixin):
 
     throttled_update_sig = pyqtSignal()  # emit from thread to do update in main thread
 
-    def __init__(self, tx: Transaction, *, parent: 'ElectrumWindow', prompt_if_unsaved: bool, external_keypairs=None):
+    def __init__(
+        self,
+        tx: Transaction,
+        *,
+        parent: 'ElectrumWindow',
+        prompt_if_unsaved: bool,
+        external_keypairs=None,
+        payment_identifier: 'PaymentIdentifier' = None,
+    ):
         '''Transactions in the wallet will show their description.
         Pass desc to give a description for txs not yet in the wallet.
         '''
@@ -403,6 +419,7 @@ class TxDialog(QDialog, MessageBoxMixin):
         self.main_window = parent
         self.config = parent.config
         self.wallet = parent.wallet
+        self.payment_identifier = payment_identifier
         self.prompt_if_unsaved = prompt_if_unsaved
         self.saved = False
         self.desc = None
@@ -537,7 +554,7 @@ class TxDialog(QDialog, MessageBoxMixin):
         self.main_window.push_top_level_window(self)
         self.main_window.send_tab.save_pending_invoice()
         try:
-            self.main_window.broadcast_transaction(self.tx)
+            self.main_window.broadcast_transaction(self.tx, payment_identifier=self.payment_identifier)
         finally:
             self.main_window.pop_top_level_window(self)
         self.saved = True