Add example for indeterminate progress bar animation

While this is not really a bug fix it was not documented
how you could create your own animated progress bar.

Task-number: QTBUG-37434

Change-Id: I5419c2f886c98189207f4e2682e8dda8206677ee
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>