Previously, when displayed content is hidden by the reveal widget there
was an optimisation such that the content was retained in the DOM but
hidden using CSS, so that it could be shown again quickly.
It turns out that a sideeffect of that optimisation is that clicking
through all the sidebar tabs leaves them all active, so that they all
need to be refreshed whenever a character is typed in an editor.
This commit suppresses the optimisation, so that hidden content is
removed from the DOM and the render tree.
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.
