stack: GtkStack:interpolate-size should be read/write

It looks like the param spec for interpolate-size was
copied from the line above it, which is a read only property.

There is a setter for interpolate-size, and it is implemented in
set_property().

gtk/gtkstack.c

@@ -470,7 +470,7 @@ gtk_stack_class_init (GtkStackClass *klass)
   stack_props[PROP_INTERPOLATE_SIZE] =
       g_param_spec_boolean ("interpolate-size", P_("Interpolate size"), P_("Whether or not the size should smoothly change when changing between differently sized children"),
                             FALSE,
-                            GTK_PARAM_READABLE);
+                            GTK_PARAM_READWRITE);
 
 
   g_object_class_install_properties (object_class, LAST_PROP, stack_props);