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.
Colorful Tabs now uses the icon of a page as the basis for the
tab color, if it has one, otherwise falls back to the previous
address based hash.
The forground color is also adjusted for dark colored tabs.
