Better support for multiple major versions of Electron

We currently only store one version of the default Fiddle editor content and the Show Me demos. However, since Fiddle allows you to use older versions of Electron, this code is not guaranteed to work well across major version API changes. As the Electron framework continues to evolve, this will only become more problematic.

We have in the past included stopgaps for API changes (that handle Electron 1 code), but nothing sophisticated enough to support the ever-changing API.

Ref #362 that we haven't merged because the API was only introduced in Electron 3.