systray: Disable broken smooth scroll with Xfwm4
It actually works in some cases, like double finger scroll on touchpads,
but it doesn't seem possible to identify these cases a priori to specify
or not the GDK_SMOOTH_SCROLL_MASK
flag. So it's better to remove it
until the upstream bug is solved.
Fixes: #411 (moved)
Edited by Gaël Bonithon