Ensure that all the children get the new stylesheet set on a parent

Ensure that all the children get the new stylesheet set on a parent

When a stylesheet was set on a parent widget then in some cases it would
not get applied to all the child widgets.  This was because the order of
the children list may have been modified while it was being set on
children.  By making a copy of the list we prevent this from being a
problem.

Task-number: QTBUG-26321

Change-Id: Iea6bf72c69a0c39746f7ef5e7893dda5a93ed7e5
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>