layoutAboutToBeChanged must be called before
persistentIndexList as the user might create persistent indexes
as a response to the signal
Fixes: QTBUG-93466
Pick-to: 6.2 5.15
Change-Id: I73c24501f536ef9b6092c3374821497f0a8f0de4
Reviewed-by: David Faure <david.faure@kdab.com>