If the platform plugin does not support the features needed
for QQuickWidget, then don't try to use it. This is a minimal
change to stop applications from crashing: it does not give
the application a way to find out if QQuickWidget is supported,
since that would mean new API, which we can't do in a patch
release.

Task-number: QTBUG-38268
Change-Id: I975a03b105b1d5c21a1d8ae440a5802ce8c1967b
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>