Button: Remove extra margins on Mac

Ideally, we would fix that in QMacStyle, but it's too much of a change
in the style right now. The values have been choosen to look good with
ComboBox, which is the other button shaped control.

Gallery example updated with 'right' sizes for buttons in first tab.

Change-Id: I331eff4930b22cf4b42cdbd6b3191fff403ddae3
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>