Prevent subclassing of gestures

For now, at least. We do this by hiding the instance and
class structures in private headers.

gtk/gtkgesturedrag.c

@@ -31,7 +31,8 @@
  * gtk_gesture_drag_get_start_point().
  */
 #include "config.h"
-#include <gtk/gtkgesturedrag.h>
+#include "gtkgesturedrag.h"
+#include "gtkgesturedragprivate.h"
 
 typedef struct _GtkGestureDragPrivate GtkGestureDragPrivate;
 typedef struct _EventData EventData;