[gtk-list] bug found in RadioMenuItem

[Karl Nelson <kenelson ece ucdavis edu>]

There appears to be a bug in GtkRadioMenuItem.  If you
create the item individually and then set the group
with gtk_radio_menu_item_set_group, more than 1 item
will be active.  

Since the code seems to assume only one will be active in
any one time the group completely fails to function.

The solution is simple when setting group an item should be 
set to not active unless it is the current owner of the group.

(RadioButton does not have this bug.)

--Karl