Refactoring private browsing menu action

IMHO, it is unnecessary to have "Open new private browsing window" in private browsing session when "Open new window" does exactly the same.

Also, it would be nice thing to have Private browsing menu action on the reach of hand, in the main menu of super button.

![pic1](https://f.cloud.github.com/assets/913292/358013/8b858faa-a134-11e2-9f63-cbf3e22a200d.png)

![pic2](https://f.cloud.github.com/assets/913292/358015/90db16fa-a134-11e2-9819-36e156caa43b.png)