Tabs are not destroyed on closing popup Window

If there is a popup Window which we close the Tabs and its Items won't be destroyed until
the main Window is closed as well.
In the attached example we can still use the Item after closing the Window but it doesn't have a valid QmlContext.
When the Window is closed all of its children's QmlContext is invalidated, which means that its parent
and QmlEngine is set to 0.
This patch seems to fix the problem: https://codereview.qt-project.org/#/c/65800/
It was reverted by: https://codereview.qt-project.org/#/c/74742/