Mouse back forward

This is my "attempt" at mapping the mouse back and forwards buttons (buttons 8 and 9) to the `<X1Mouse>` and `<X2Mouse>` mappings (as in VIM).

I seem to have become stuck as catching the `mouse-button-event` signal does not repropagate, so while I can handle the forwards and backwards buttons the "normal" mappings do not work.

The `dwb` browser seems to explicitly handle all the buttons: https://gist.github.com/dantleech/64c818a15a41fe0d8893

Is there a way to pass the event back to webkit?