css: Rewrite selectors

css: Rewrite selectors

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.