MainWindow: Fix the wrong initial value of the Lock QAction when it is the first launch

src/Gui/ui/MainWindow.ui

@@ -382,6 +382,9 @@
    <property name="checkable">
     <bool>true</bool>
    </property>
+   <property name="checked">
+    <bool>true</bool>
+   </property>
    <property name="text">
     <string>Lock</string>
    </property>