Add QQuickWidget::setClearColor()

And make qquickviewcomparison able to demonstrate how to create a
partially transparent QQuickWidget.

Avoid also recreating the QQuickView/QQuickWidget multiple times
when switching between the radio buttons.

[ChangeLog] Added QQuickWidget::setClearColor() in order to support
semi-transparent QQuickWidgets.

Change-Id: I319ad4afbe909530274d09f2a7fcff23730d6ebd
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>