evdev: Move additional pointer buttons after the old 4-7 scrolling ones
evdev: Move additional pointer buttons after the old 4-7 scrolling ones

On X11 those were skipped, so additional pointer buttons would come up
as button >= 8 events. Do here the same, so we remain compatible across
backends.

https://bugzilla.gnome.org/show_bug.cgi?id=758237

GNOME/clutter,GNOME/clutter,Distrotech/clutter,Distrotech/clutter,Distrotech/clutter,GNOME/clutter,GNOME/clutter,Distrotech/clutter,Distrotech/clutter,Distrotech/clutter,GNOME/clutter