Callers can persist a QModelIndex which was not persisted before in a
slot connected to the signal, and such a persisted index must be updated
in the course of the layoutChange.
Store the indexes to persist after emitting the signal.
Task-number: QTBUG-32981
Change-Id: Ibee4c0d84817d72603a03fe5b22fdeefeac0695e
Reviewed-by: David Faure <david.faure@kdab.com>