Avoid extra platformwindows with QQuickWidget

QQuickWidget is nice but does not work on platforms like eglfs
because it always creates a (hidden) platform window for the
QQuickWindow.

This is now fixed by avoiding calling create() on the window and
using the toplevel window of the QQuickWidget instead.

Change-Id: Ia552f7a16d8b913fb798fde04b9304c6d037a46c
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>