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