There are GtkGestureSingle subclasses that can be made to handle multiple
fingers (GtkGestureSingle is a subclass of GtkGesture, and not the
opposite, after all). And GtkGestureSwipe already tries to handle
GDK_TOUCHPAD_SWIPE events, except this event handler silently ignores
those.

Falling back to the GtkGesture generic handler which already
handles touchpad gesture events fixes this.