Make QQuickRenderControl usable (still private)

QQuickRenderControl needs a QQuickWindow in order to function. This
required the use of a private QQuickWindow constructor, meaning that
only friend classes of QQuickWindow could use QQuickRenderControl.

This change adds a factory function, QQuickRenderControl::createOffscreenWindow().

Task-number: QTBUG-38996
Change-Id: I92f6f1f60045934f3dd33c300352247852463c73
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>