QAIV: speed up isRowHidden for the case where no rows are hidden.

isPersistent(index) is a linear search, which we can avoid altogether
(as well as the creation of a persistent model index, is idx is indeed persistent)
when the row-hiding feature isn't used at all.

Change-Id: I07488dddf56e299806cc29749e633da00115fc30
Reviewed-by: Jan Arve Sæther <jan-arve.saether@theqtcompany.com>