GtkOrientable use G_PARAM_EXPLICIT_NOTIFY

This one is a little tricky, since we override this property
in many places, and you cannot add flags when overriding. So,
all places where this is overridden will have to make sure to
notify explicitly.

gtk/gtkorientable.c

@@ -62,7 +62,7 @@ gtk_orientable_default_init (GtkOrientableInterface *iface)
                                                           P_("The orientation of the orientable"),
                                                           GTK_TYPE_ORIENTATION,
                                                           GTK_ORIENTATION_HORIZONTAL,
-                                                          GTK_PARAM_READWRITE));
+                                                          GTK_PARAM_READWRITE|G_PARAM_EXPLICIT_NOTIFY));
 }
 
 /**