Better QWindowContainer by not relying on native widgets.

We change the behavior slightly from the initial implementation in
5.1. Forcing the use of native child widgets is causing massive
performance issues so instead, we attach the embedded QWindow directly
to the root window. The only exception is QScrollArea and QMdiArea
which still enforces native windows for the entire parent chain
to make clipping and stacking work.

Task-number: QTBUG-34138
Change-Id: If713637bd4dce630552ace2f8ad6b2e86c063721
Reviewed-by: Lars Knoll <lars.knoll@digia.com>