Make scroll events work again in GTK3.

Make scroll events work again in GTK3.

Apparently, if you don't specify GDK_SMOOTH_SCROLL_MASK in a widget's
event mask, then you don't receive "scroll_event" signals at all, even
of the non-smooth variety that was all GTK2 had. Hence, neither mouse
scroll wheels nor touchpad scroll gestures seem to generate any
response.

Adding GDK_SMOOTH_SCROLL_MASK brings the old scroll events back again,
so this is at least no worse than GTK2 was. But in GTK3 we _ought_ to
be able to do better still, by supporting smooth scrolling from
touchpads; this commit doesn't do that.