Alist of face attributes.
The elements are of the form (KEY TYPE PRE-FILTER POST-FILTER),
where KEY is the name of the attribute, TYPE is a widget type for
editing the attribute, PRE-FILTER is a function to make the attribute's
value suitable for the customization widget, and POST-FILTER is a
function to make the customized value suitable for storing. PRE-FILTER
and POST-FILTER are optional.
The PRE-FILTER should take a single argument, the attribute value as
stored, and should return a value for customization (using the
customization type TYPE).
The POST-FILTER should also take a single argument, the value after
being customized, and should return a value suitable for setting the given face attribute.