We now instantiate all the generated web API constructors and expose
them on the window object. We also set the generated prototypes on
instantiated wrappers.
Also, we should obviously find a way to generate this code. :^)
This patch adds a FooPrototype and FooConstructor class for each IDL
interface we generate JS bindings for.
These classes are very primitive and don't do everything they should
yet, but we have to start somewhere. :^)
Work towards #4789
