Previously we kept a Selector object for every "simple selector" (term
from CSS spec). Now we keep one for every match operation. So given the
selector
".a b:focus"
we will have 4 elements:
- pseudoclass ":focus"
- element "b"
- match any desendant (the space)
- class ".a"
Each of those is represented by a "selector class" which is basically
the collection of vfuncs for this selector.
