widget: handle smooth-scroll events

With touchpads and similar devices, scroll events are not emitted on
Wayland, but only smooth-scroll events.

There is some discussion about handling smooth-scroll events in Spice
(see "Add horizontal mouse wheel support" thread), but it will mean
support from various components. For compatibility reasons, in the
meantime, let's synthetize the smooth-scroll events to regular scroll
buttons to fix scrolling.

Tested on f28, gnome-shell under X and wayland.

Related to:
https://bugzilla.redhat.com/show_bug.cgi?id=1386602



Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Christophe Fergeau <cfergeau@redhat.com>