Use native handles for parent change check in QWindowsWindow.

Use native handles for parent change check in QWindowsWindow.

QWindow::setParent() sets the parent to zero instead of the desired
parent, if platform window has not yet been created for the parent.
This caused QWindowsWindow::setParent() to skip setting the parent
later, when correct window was specified, as the QWindow parent-child
relationship hadn't changed. Fixed by changing the the check to use
native handles instead.

Task-number: QTBUG-26791
Change-Id: I292a1ddf746583a7268f2d07c20166995c0dd7d6
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>