realtime plugin: allow str for JsCode arg

[Conengmo]

Idea split off from #1856.

For defined keyword arguments that expect a Javascript function, allow not only the new JsCode class, but also str. Adapt JsCode to allow both str and JsCode arguments for easier instantiation.

This means that users can use these keyword arguments without having to deal with JsCode. And JsCode is only necessary for undefined keyword arguments that are Javascript functions.

Upside of this is that users have less classes to worry about when using this plugin. Downside is it may be confusing that using a string doesn't work for undefined keyword arguments that are Javascript functions. But since that's a more advanced move, I hope those users can read and understand the docstring.