QHeaderview - many sections removed performance boost.

QHeaderview - many sections removed performance boost.

When the QHeaderView model rowCount is decreased then the headerview
needs to remove some sections.

If e.g swapSections had occured then qheaderview before this patch did
a very slow removal by removing one section at a time and for each section
it restored visual and logical indexes.

By temporarily storing the logical index on the section we
can reduce the execution time of this a lot. The old code is only
faster when we remove one index (and is kept for that situation)

There is a complexity in difference (and many factors in our
qheaderview benchmark (the trunc part))

Change-Id: If45eab17fbca3364d458b004980570b5d013e4bd
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>