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
This patch does a number of things, unfortunately all at once:
* Hoists a large subset of the layout wrapper functionality into the style system.
* Merges TElementAttributes into the newly-created TElement.
* Reorganizes LayoutData by style vs layout, and removes LayoutDataShared.
* Simplifies the API for borrowing style/layout data.
There's still more to do to make the style system usable standalone, but
this is a good start.
