add gtk_radio_button_join_group method for bindings

* this mirrors the committed change for gtk_radio_action_join_group in
  commit 85b53969b2
* Due to object ownership issues it is impossible to correctly use
  get_group/set_group from a GI binding
* join_group is safer because at the binding level it works with individual
  GtkRadioButton objects and not with the list of objects that gets
  modified in the library

https://bugzilla.gnome.org/show_bug.cgi?id=628935

gtk/gtkradiobutton.c

@@ -346,6 +346,61 @@ gtk_radio_button_set_group (GtkRadioButton *radio_button,
   g_object_unref (radio_button);
 }
 
+/**
+ * gtk_radio_button_join_group:
+ * @radio_button: the #GtkRadioButton object
+ * @group_source: (allow-none): a radio button object whos group we are 
+ *   joining, or %NULL to remove the radio button from its group
+ *
+ * Joins a #GtkRadioButton object to the group of another #GtkRadioButton object
+ *
+ * Use this in language bindings instead of the gtk_radio_button_get_group() 
+ * and gtk_radio_button_set_group() methods
+ *
+ * A common way to set up a group of radio buttons is the following:
+ * |[
+ *   GtkRadioButton *radio_button;
+ *   GtkRadioButton *last_button;
+ *
+ *   while (/* more buttons to add */)
+ *     {
+ *        radio_button = gtk_radio_button_new (...);
+ *
+ *        gtk_radio_button_join_group (radio_button, last_button);
+ *        last_button = radio_button;
+ *     }
+ * ]|
+ *
+ * Since: 3.0
+ */
+void
+gtk_radio_button_join_group (GtkRadioButton *radio_button, 
+			     GtkRadioButton *group_source)
+{
+  g_return_if_fail (GTK_IS_RADIO_BUTTON (radio_button));
+  g_return_if_fail (group_source == NULL || GTK_IS_RADIO_BUTTON (group_source));
+
+  if (group_source)
+    {
+      GSList *group;
+      group = gtk_radio_button_get_group (group_source);
+
+      if (!group)
+        {
+          /* if we are not already part of a group we need to set up a new one
+             and then get the newly created group */
+          gtk_radio_button_set_group (group_source, NULL);
+          group = gtk_radio_button_get_group (group_source);
+        }
+
+      gtk_radio_button_set_group (radio_button, group);
+    }
+  else
+    {
+      gtk_radio_button_set_group (radio_button, NULL);
+    }
+}
+
 /**
  * gtk_radio_button_new:
  * @group: an existing radio button group, or %NULL if you are creating a new group.

gtk/gtkradiobutton.h

@@ -86,7 +86,8 @@ GtkWidget* gtk_radio_button_new_with_mnemonic_from_widget (GtkRadioButton *radio
 GSList*    gtk_radio_button_get_group                     (GtkRadioButton *radio_button);
 void       gtk_radio_button_set_group                     (GtkRadioButton *radio_button,
                                                            GSList         *group);
-
+void            gtk_radio_button_join_group        (GtkRadioButton        *radio_button,
+                                                    GtkRadioButton        *group_source);
 G_END_DECLS
 
 #endif /* __GTK_RADIO_BUTTON_H__ */