Revert background to Rectangle as it was in previous versions of this file.

This gives the bar a white background, instead of a transparent background.

CURA-8688

resources/qml/MainWindow/ApplicationMenu.qml

@@ -21,7 +21,7 @@ Item
         id: applicationMenu
         width: parent.width
         height: UM.Theme.getSize("context_menu").height
-        background: Item {}
+        background: Rectangle {}
 
         FileMenu {}
         EditMenu {}