QMacStyle: fix auto-default button animation

Make sure to stop any previously active (auto-default button)
animation when the auto-default button changes.

Task-number: QTBUG-31668
Change-Id: Iaa9bfa485b11e49cd7f765d0e54b4e6391ad2f6d
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>