QHeaderView::moveSection performance boost

The patch also speeds up swapSections and hideSection a lot.

It work by eliminating the Span. (That is forcing each 'Span'
to have exactly one element) That saves a lot of loops since
we can often lookup info fast - and/or change it fast.

Since it is often a complexity change, it is difficult to
put %-increase on. (The most used linear function is
recalcSectionStartPos() - and it has a very low constant)

However comparing with the new benchmark (2500 rows)
swapSection, showHideSection and moveSection are about
20-40 factors faster. (Yes, it is a lot faster!)

In the benchmark moveSection is about 300 factors faster.

Beside being a far better model it is also far more simple.

This fix partly solves:
Task-number: QTBUG-19092

Change-Id: I8deeb9315276d15c68e8a27d5dcb8e0c0badf367
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

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