wayland: Implement touchpad gesture events

On wayland, the gestures protocol defines a wl_pointer_gestures global
object, that will match in number with wl_seats, swipe and pinch
interfaces can be obtained from it, which events are translated into
GdkEventTouchpadSwipe/Pinch events.

gdk/wayland/gdkdisplay-wayland.h

@@ -71,6 +71,7 @@ struct _GdkWaylandDisplay
   struct wl_input_device *input_device;
   struct wl_data_device_manager *data_device_manager;
   struct wl_subcompositor *subcompositor;
+  struct _wl_pointer_gestures *pointer_gestures;
 
   GList *async_roundtrips;