Site-patching possibility for Safari

Safari's extension API doesn't provide a way to intercept requests
initiated by plugins, so those cases need special care (or at least the
popular sites).

This commit adds a new JS file (sitepatch-safari.js), which will store the
patches (if it's possible to create one) for specific sites.

As an example, this commit includes a technique for removing in-video ads
from YouTube videos.
Use different page navigation detection for Safari

The beforeNavigate event wasn't reliable (sometimes didn't fire, sometimes
fired unnecessary when opening a link with the middle click - which had a
workaround previously, but that's also removed by this commit).

When the event didn't fire, the bindTabToPageStats method didn't run,
and the requests related to the tab weren't blocked.