Rules kcm: Use new property ComboBox.currentValue

Summary:
Use the properties `valueRole` and `currentValue`, introduced in Qt5.14 for `QQC2.ComboBox`

This didn't work before due to QQC2 version being mis-detected at build time, so the code mocked an internal `currentValue` property.

After D28859, it is now required to fix:
   "file:///home/nate/kde/usr/share/kpackage/kcms/kcm_kwinrules/contents/ui/RulesEditor.qml"
   "Error loading QML file.\n42: Type RuleItemDelegate unavailable\n68: Type OptionsComboBox unavailable\n35: Cannot override FINAL property\n"
   QCoreApplication::postEvent: Unexpected null receiver

Test Plan:
Try to edit a rule or add a new one.
The rule editor page shows.

Reviewers: ngraham, #kwin, zzag

Reviewed By: ngraham, #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29145

kcmkwin/kwinrules/package/contents/ui/OptionsComboBox.qml

@@ -29,15 +29,7 @@ QQC2.ComboBox {
     id: optionsCombo
 
     textRole: "display"
-    //TODO: After KF5 (qqc2-desktop-style) depends on Qt 5.15 this can be simplified using newer API
-    //      (https://bugs.kde.org/show_bug.cgi?id=419521)
-    //      valueRole: "value"
-    property var currentValue
-
-    onActivated: (index) => {
-        var modelIndex = model.index(index, 0);
-        currentValue = model.data(modelIndex, Qt.UserRole);
-    }
+    valueRole: "value"
 
     property bool multipleChoice: false
     property int selectionMask: 0