qt status: display "loading wallet" temporarily

this will likely only be visible for large wallets;
it gets overwritten by update_status()

electrum/gui/qt/main_window.py

@@ -1962,7 +1962,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, PrintError):
         sb.setFixedHeight(35)
         qtVersion = qVersion()
 
-        self.balance_label = QLabel("")
+        self.balance_label = QLabel("Loading wallet...")
         self.balance_label.setTextInteractionFlags(Qt.TextSelectableByMouse)
         self.balance_label.setStyleSheet("""QLabel { padding: 0 }""")
         sb.addWidget(self.balance_label)