MIDORI_VERSION_SUFFIX resolves to a string that needs to be
added to each extension's version at build time. If it is
different than the running Midori's, it is shown in the
extension list.
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.
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.