Getting rid of the old widget mechanism files finally gives us a payoff
for all the refactoring. Still a bit of tidying up to do, and we need
to re-introduce the animation mechanisms.
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.
