Port away from QtScript

QtScript is deprecated and not available in Qt6

I tried porting the scripting to QJSEngine, but found two issues:

• QJSEngine doesn't have a debugger interface like QtScript has: https://bugreports.qt.io/browse/QTBUG-55905
• QJSEngine doesn't support dynamic properties: https://doc.qt.io/qt-6/qjsengine.html#dynamic-qobject-properties