A dynamically-created Window can have a parent Item and vice-versa

A dynamically-created Window can have a parent Item and vice-versa

There can be a QML-declared Item which uses Component.createObject to
instantiate a Window; in that case the Window will be transient for
the window containing the Item.  There can also be a QML-declared
Window which uses Component.createObject to instantiate an Item;
in that case the Item's parent will be set to the Window's contentItem.

Task-number: QTBUG-33644
Change-Id: I0b1fe2e98c862c100e52bd5952788af3a727d25e
Reviewed-by: Alan Alpert (Personal) <416365416c@gmail.com>