QMacStyle: Fix QToolButton appearance on Yosemite

On Yosemite checkable toolbuttons look differently
when highlighted - this modification tries to imitate such look
without pixmaps we used on OS X >= 10.5 && < 10.10.

Change-Id: I0fcdff42a67fa6d5188c9dc64401f0f27ae60b5d
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>