Binding does not restore original property value if it was assigned as a constant

The Binding type only restores a property's original value if was set to an expression, and not a constant value.

Take the following example:

import QtQuick 2.0

Rectangle {
    id: root

    property int count
    property bool enableBinding: true

    width: 200; height: 200

    color: "blue"           // Doesn't work, this color won't be restored when enableBinding=true
    //color: true ? "blue" : "blue"  // Works, this color will be restored when enableBinding=true

    Timer {
        interval: 800
        running: true
        repeat: true
        onTriggered: root.count += 1
    }

    Binding {
        id: binding
        target: root
        property: "color"
        value: count % 2 ? "green" : "yellow"
        when: enableBinding
    }

    focus: true
    Keys.onSpacePressed: {
        enableBinding = !enableBinding
    }
}

The Rectangle's color is initially set to blue, but the Binding object causes the Rectangle's color to alternate between green and yellow.

When enableBinding=false, this binding is disabled, and the original blue color should be restored. However, this only happens if the initial color property value was set as an expression (e.g. "if (1) "blue") and not if it was set as a constant (e.g. "blue").

This makes the Binding type unusable in the case where a library component needs to temporarily modify a provided object, as there is no guarantee that the object's original property value will be restored.