qt: don't flash QWidgets on startup before main window is visible

Consider wallet without password set. Using Qt GUI.
When starting the app, before the main window appears, small artefacts
("minimised" windows?) would appear very briefly and then disappear.

electrum/gui/qt/invoice_list.py

@@ -58,8 +58,10 @@ class InvoiceList(MyTreeView):
             item[3].setFont(QFont(MONOSPACE_FONT))
             self.addTopLevelItem(item)
         self.selectionModel().select(self.model().index(0,0), QItemSelectionModel.SelectCurrent)
-        self.setVisible(len(inv_list))
-        self.parent.invoices_label.setVisible(len(inv_list))
+        if self.parent.isVisible():
+            b = len(inv_list) > 0
+            self.setVisible(b)
+            self.parent.invoices_label.setVisible(b)
 
     def import_invoices(self):
         import_meta_gui(self.parent, _('invoices'), self.parent.invoices.import_file, self.update)

electrum/gui/qt/request_list.py

@@ -67,6 +67,7 @@ class RequestList(MyTreeView):
     def update(self):
         self.wallet = self.parent.wallet
         # hide receive tab if no receive requests available
+        if self.parent.isVisible():
             b = len(self.wallet.receive_requests) > 0
             self.setVisible(b)
             self.parent.receive_requests_label.setVisible(b)