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/layout/data.rs

@@ -4,7 +4,7 @@
 
 use construct::ConstructionResult;
 use incremental::RestyleDamage;
-use style::data::PrivateStyleData;
+use style::servo::PrivateStyleData;
 
 /// Data that layout associates with a node.
 pub struct PrivateLayoutData {