QSqlQuery::isNull string overload

Introduce isNull overload to take field name as a parameter.

This is corresponding to the commit
7e6e1412348fb8d8df844d821ee80d6d3de69517

Change-Id: I122f79707d26eaa09c2f38dc31aeee1dac7de33b
Reviewed-by: Mark Brand <mabrand@mabrand.nl>
add mising tag to method documentation

Change-Id: Iafd7a268bf3b3be9a06ab1badc23e3db52df9586
Reviewed-by: Mark Brand <mabrand@mabrand.nl>
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>