Return value for atk_action_get_keybinding

["Padraig O'Briain" <Padraig Obriain Sun COM>]

I think that the G_CONST_RETURN should be removed from the return value for 
atk_action_key_keybinding(); i.e. the value returned should be freed by the 
caller.

This suggestion is based on my experience attempting to implement this function 
for the "activate" action for a menu item.

Padraig