This introduces the need for a grab function in QQuickWidget.
The render control has one already so there is no reason for not exposing
this in QQuickWidget too.

This also means that a relatively meaningful autotest can be now be added.

[ChangeLog][QtQuick] Added QQuickWidget::grabFramebuffer() for capturing the content into a QImage.

Task-number: QTBUG-37589
Change-Id: I5ca8192c0ef8dab4f076a4db27b64aebe3359bb8
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>