Correct QToolBar layout on retina displays.

QStyleOptionToolButton::iconSize will be 2x on retina
displays. The layout size should not be 2x however,
that causes excessive spacing in the toolbar.

Task-number: QTBUG-34484
Change-Id: I48cf47044af5b76f39e08c1d580034b84ef2755d
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>