Take into account icon size when determining button size

When a style calculates the sizeFromContents on a pushbutton it needs to
be informed about the size of the icon on the button.

In oxygen icons with buttons are slightly larger than buttons without.

Change-Id: I2f2d5f08747ad5e6ac0dfb50531c3e7627e9b35f
Reviewed-by: Sebastian Kügler <sebas@kde.org>
Reviewed-by: Marco Martin <mart@kde.org>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>