Is the JavaScript code to configure buttons documented? I can't seem to find any documentation.

For example, I notice in a few example that we can set the icon on a button with:

this.set_imagename("openfolder");

I am guessing this works because RTE knows what the "openfolder" icon is but what if I want a custom icon? Is it possible to do something like:

this.set_image("http://mysite.com/images/myimage.jpg");