Bug: 10372

With compositor enabled, horizontal scrolling via the mousewheel or
touchpad would adjust the window opacity.

Apparently, this is confusing so make it optional and disabled by
default.

If you fancy this feature, you can re-enable it by setting
"horiz_scroll_opacity" to true in xfconf.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>