Due to the change of the stock icon names in GTK+ 2.22 the
stock item trick doesn't work anymore. Using the authentication
icon directly is good enough.
Iterating an array by a GList is considerably faster than
continuously retrieving items, however it is also a lot
more complicated. So the new macro takes care of that and
uses a new semi-private function to avoid copying the list.
Note that the macro can't be nested, which basically isn't
useful in practise anyway.
Split the code a bit more and use one GtkTreeStore for all
CookieManagerPage instances so we save some cycles when updating
the store when cookies have changed.
