Implement pop-up blocking for Safari

It works similarly to the xhr intercepting, except here the window.open
global function is being overridden.
Note that it could only work if the site's Content Security Policy allows
inline scripts, and the script on the webpage doesn't have a copy of the
original window.open function (it can happen only if the page has an
inline script in its head element, where the reference to the original
function can be obtained - likely this cannot be prevented in Safari).