Move special cases of CSS 'display' the property to computed value, per spec.
Use Au::scale_by instead of duplicating it.
Fix selector parsing.
Move CSS21 pseudo-elements parsing out of parse_simple_pseudo_class
Implementation of pseudo class 'first-child'.
auto merge of #1143 : SimonSapin/servo/selector-parsing+first-child, r=SimonSapin

Fixes #1133, fixes #1134.

This is a rebase and clean-up of these two pull requests that have been reviewed individually. Additionally, 5c59699 disables parsing of `display: list-item` which is not supported yet and may have been the cause of the hangs when testing #1134.