status: preserve horizontal scroll across updates

When a file name is too long, a horizontal scroll bar appears in the status widget. As the selection is updated, `QTreeView` tries to put the first column back in 'full view' and thus resets the scroll bar horizontal position to `0` [(source)](http://doc.qt.digia.com/4.6/qabstractitemview.html#autoScroll-prop)

This commit disables that behaviour.
`setAutoScroll(..)` is available as early as in Qt 4.0.