Map mouse forward and back correctly on X11

Buttons 6 and 7 are for horizontal scrolling, just like buttons 4 and 5
are for vertical scrolling. I couldn't find a list of button numbers in
the X11 protocol specs, but this is what the buttons are bound to on my
Logitech G502 and
http://xahlee.info/linux/linux_x11_mouse_button_number.html also
confirms this.

src/x11/window.rs

@@ -684,8 +684,8 @@ fn mouse_id(id: u8) -> MouseButton {
         1 => MouseButton::Left,
         2 => MouseButton::Middle,
         3 => MouseButton::Right,
-        6 => MouseButton::Back,
-        7 => MouseButton::Forward,
+        8 => MouseButton::Back,
+        9 => MouseButton::Forward,
         id => MouseButton::Other(id),
     }
 }