Add mouse side-buttons navigation

[Arskan17]

When pressing the mouse's side-buttons, it should either load the previous page (if canGoBack ?), or the next page (if canGoForward ?).
I think it can be done in app/ui/omni-box.js, but I'm not sure. Might need to tinker around and see...

It would be cool to have this feature, like in many other browsers.
Instead of having to press the forward/back buttons while navigating, mouse buttons could be used.