Fix incorrectly scaled constants on Mac

Fix incorrectly scaled constants on Mac

This fixes several scaled constants on mac that are inherited from
commonstyle. (such as toolbutton arrows) It is probably easiest
to see when running Windows style on mac.
The problem was that the code assumed a default dpi of 96 would be
used on all platforms.

Change-Id: I83789589009b268dcb1d96629c3ec9e8f968a891
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>