[DEMO: Panels] Fix element subclassing in c.inherit() to work in Gecko

[slightlyoff]

We currently rely on some proto magic with regards to existing DOM elements to provide the illusion of subclassing HTML elements. Combined with Mutation Observers, this allows us to fake HTML subclassing in the panels demo. We need a way to do the imperative part of this in Gecko (perhaps by routing through the parser first?)