[applets/keyboardlayout] Fix config hookup

By using the automagic kcfg system we have the apply button work
correctly.

applets/keyboardlayout/contents/ui/configGeneral.qml

@@ -4,10 +4,10 @@ import QtQuick.Layouts 1.12
 import org.kde.kirigami 2.5 as Kirigami
 
 Kirigami.FormLayout {
+    property alias cfg_showFlag: showFlagCheckBox.checked
 
     CheckBox {
-        checked: plasmoid.configuration.showFlag
-        onCheckedChanged: plasmoid.configuration.showFlag = checked
+        id: showFlagCheckBox
         text: i18n("Show flag")
     }
 }