Add hint API to QAIM::layout*Change signals.

Add hint API to QAIM::layout*Change signals.

Usually layoutChange is used for sorting elements. The parents parameter
to the signal already hints at a localization of what is being sorted, if
it's just one level in the tree. The new hints enum can be used to only
consider how the first column of a table gets sorted, because all other
columns will be sorted the same way.

Change-Id: I65dd2996894bbdb45a2f6288edbeaa49e4053256
Reviewed-by: David Faure <faure@kde.org>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>