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>

Besides serving as a side-by-side test tool for QQuickView and
QQuickWidget (including multisampling), it also demonstrates some
useful practices for rendering 3D content via QQuickFramebufferObject.

Done-with: Paul Olav Tvete <paul.tvete@digia.com>
Change-Id: Ie73e998ee91e32ef1535dd6f0f65c8a69addcc5e
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>