No keyframes list properties

Explicit use of the Timeline::keyframes and KeyframeGroup::keyframes list properties is not supported:

Timeline {
    keyframes: [ // not supported
        KeyframeGroup {
            keyframes: [ // not supported
                Keyframe {}
            ]
        }
    ]
}

Instead, place KeyframeGroup and Keyframe directly into the parent object:

Timeline {
    KeyframeGroup {
        Keyframe {}
    }
}

This syntax is supported in both Qt Quick Ultralite Timeline and Qt Quick Timeline.