Custom toolbar button for versions

[wl606]

I am doing API documentation, and need to have separate help systems for each version of the API. What I want to do is add a custom toolbar button that allows the user to switch between versions. I think conceptually, it is identical to the switch language button. However, I haven't been able to locate the exact Javascript that handles that behavior in order to mimic it with for the version switch button, and my Javascript is pretty rusty. It is just a simple window.open ?