Add the RoundedRectangle as background to the ExpandableComponent

This way the expandable components can have rounded corners only on one side,
thus preventing the need to do add backgrounds to the rows that they are in.

CURA-5785

resources/qml/Cura.qml

@@ -259,6 +259,7 @@ UM.MainWindow
                     onHideTooltip: base.hideTooltip()
                     width: UM.Theme.getSize("print_setup_widget").width
                     height: UM.Theme.getSize("stage_menu").height
+                    headerCornerSide: 4 // Show corners on the right side
                 }
             }