The default button has different text color on OS X 10.10, and
doesn't pulsate anymore.

We first make sure we don't start any animation for default buttons.
Then, we set the right text color in the attached palette, which
implies making a copy of the style option object. This increases the
size of this patch but keeps things sane. (It's also more concise than
the QGtkStyle approach that consists on duplicating QCommonstyle's
code for rendering CE_PushButtonLabel).

Task-number: QTBUG-40833
Change-Id: Ifac118bb607eec17400c6ae42342a223a40843b4
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>