Get rid of the separate renderContext stack and instead have a parent
pointer on renderer nodes. This lets us walk back up the render tree to
resolve context references
This arrangement takes better advantage of the similarities between the
now deleted widget renderer and the element renderer. It also obviates
the need for wrapper elements around every widget.
Thus introducing a few functional regressions, but it's going to be
easier to fix things up without the old code knocking around and
getting in the way.
