QHeaderView - move ishidden into the section

Since we already have the section there is no need to have an
extra structure. It simplifies the code itself and the code
is overall more efficient.

The benchmark seems to show overall performance increasement
various places (e.g insert, swapSection).

Change-Id: I623453b69a9a830908e8d9d5f3816ccebe073c9f
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>