This matcher always matches only on some relvant things and ignores the
rest. This allows you to match only on name and class, but ignore state
and parents/siblings for example.
We now track the position as a (type,a,b) tuple where the numbers make
up the an + b formula from CSS3 nth-child.
Also, the get_sibling() and get_sibling_index() vfuncs were replaced by
a has_position() vfunc. This is mostly so that the matcher can always
return TRUE. And I need that for the everything matcher.