Revise Qt Quick Ultralite layouts Example

A follow-up of https://bugreports.qt.io/browse/QTMCU-376

The problem with current implementation is that the default behavior looks wrong:

That all the items in GridLayout are located as the same location and overlapping each other. Users (even some of us) are confused and thinking that GridLayout is bugged.

But actually, it is not bugged. A "columns" of value 0 means "nothing happens", and GridLayout is not doing anything. Nevertheless, "doing nothing" upon demo being launched results in confusing result.

So 2 suggestions:
1. Revise that demo
2. Revise documentation too, including pointing out "-1 is the default value, i.e. no limitation" and "0 means no layout update".