Refactor style to be completely backend-independent

This commit refactors the style crate to be completely independent of
the actual implementation and pseudo-elements supported.

This also adds a gecko backend which introduces parsing for the
anonymous box pseudo-elements[1], although there's still no way of
querying them.

https://mxr.mozilla.org/mozilla-central/source/layout/style/nsCSSAnonBoxList.h

components/script/layout_interface.rs

@@ -24,7 +24,7 @@ use std::sync::mpsc::{Receiver, Sender, channel};
 use string_cache::Atom;
 use style::context::ReflowGoal;
 use style::selector_impl::PseudoElement;
-use style::stylesheets::Stylesheet;
+use style::servo::Stylesheet;
 use url::Url;
 use util::ipc::OptionalOpaqueIpcSender;