Verify that closing a QWindow using either the close()
API or a close event works as expected, and that the window
can be re-created. Also test QWindows with child windows.
Task-number: QTBUG-46701
Change-Id: I4c12452ff58e1233536c2d6932e72cf924d8ed74
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>