Update to the new Opera icon 

This patch updates `opera.png` to the latest version (i.e. the icon used in `Opera.app` for Opera v15), and removes the SVG version of the old Opera logo now that it’s not current anymore.

I didn’t run the PNG image through ImageOptim, as I was paranoid about losing quality.

(Note that http://business.opera.com/press/resources/#opera-icon still displays the old icon.)

P.S. `all-desktop.png` will need to be updated. On that note, it would be cool if we had an automated script that did this.