button: Add private api to get at the gesture

Attaching another gesture from the outside does not
work currently, so let widgets share their button's
gesture for now.

gtk/gtkbutton.c

@@ -1146,3 +1146,11 @@ gtk_button_get_icon_name (GtkButton *button)
 
   return NULL;
 }
+
+GtkGesture *
+gtk_button_get_gesture (GtkButton *button)
+{
+  GtkButtonPrivate *priv = gtk_button_get_instance_private (button);
+
+  return priv->gesture;
+}

gtk/gtkbuttonprivate.h

@@ -41,6 +41,7 @@ struct _GtkButtonPrivate
   guint          child_type            : 2;
 };
 
+GtkGesture * gtk_button_get_gesture (GtkButton *button);
 
 G_END_DECLS