qt: don't refresh invoice list per invoice_status event while syncing

Each invoice_status event makes InvoiceList.refresh_item recompute the
invoice status, which scans prevouts, on the GUI thread. While the
wallet is syncing these events arrive in bulk - one per touched
invoice per tx addition - so a wallet with many invoices sharing a
destination scriptpubkey freezes the GUI for the duration of the sync
(observed via py-spy: the GUI thread inside _is_onchain_invoice_paid
under refresh_item, while the event loop thread ran a detection pass).

While not up_to_date, just set need_update: update_wallet() already
defers update_tabs() until is_up_to_date(), and then rebuilds the
invoice list once, in one batch.

Author: sashazykov

electrum/gui/qt/main_window.py

@@ -1387,6 +1387,12 @@ def on_event_request_status(self, wallet, key, status):
     def on_event_invoice_status(self, wallet, key, status):
         if wallet != self.wallet:
             return
+        if not wallet.is_up_to_date():
+            # while syncing, these events arrive in bulk (per touched invoice per tx), and
+            # refresh_item recomputes the status, scanning prevouts on the GUI thread.
+            # update_wallet() rebuilds the list in one batch once we are up_to_date again.
+            self.need_update.set()
+            return
         if status == PR_PAID:
             self.send_tab.invoice_list.delete_item(key)
         else: