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.
There is now an inclusive_descendants_version field of each node, which
increases each time the node, or any of its descendants, is dirtied.
This can be used for cache invalidation, by caching a version number
and comparting the current version number against the cached version number.
After rebasing, this suddenly became a problem again, even though there's no actual size increase here
(we're shrinking NodeFlags by 1 byte, and adding 1 byte of EventState). Moving the NodeFlags to the end
of Node and the EventState bits to the beginning of Element doesn't seem to helper either.
This is probably a padding issue that's worth investigating at some point, but given the level
of churn in this code it doesn't seem worth it to fuss to much over this right now.
