universal access: Fix non-expanding sliders in zoom options

The behaviour of the GtkBox expand child property changed in
GTK+ 3.10, back to its traditional behaviour: It no longer
propagates up. Propagation is now reserved for the GtkWidget
hexpand property. Therefore, we can get the sliders back
to the way they were in 3.8 by setting hexpand to TRUE.

http://bugzilla.gnome.org/show_bug.cgi?id=708493