Prevent subclassing of gestures

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

gtk/gtkgesturezoomprivate.h

@@ -0,0 +1,41 @@
+/* GTK - The GIMP Toolkit
+ * Copyright (C) 2012, One Laptop Per Child.
+ * Copyright (C) 2014, Red Hat, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library. If not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author(s): Carlos Garnacho <carlosg@gnome.org>
+ */
+#ifndef __GTK_GESTURE_ZOOM_PRIVATE_H__
+#define __GTK_GESTURE_ZOOM_PRIVATE_H__
+
+#include "gtkgestureprivate.h"
+#include "gtkgesturezoom.h"
+
+struct _GtkGestureZoom
+{
+  GtkGesture parent_instance;
+};
+
+struct _GtkGestureZoomClass
+{
+  GtkGestureClass parent_class;
+
+  void (* scale_changed) (GtkGestureZoom *gesture,
+                          gdouble         scale);
+  /*< private >*/
+  gpointer padding[10];
+};
+
+#endif /* __GTK_GESTURE_ZOOM_PRIVATE_H__ */