Cocoa: Amend 5d07a4d9 to avoid an assert in QWidget::mapTo()

For now, we only call 'flushWindowSystemEvents()' for top-level
QWindows.

Change-Id: Ie6f6d3f9b70241820d9c38bcc0caf832d521268b
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>