Use the official model index for the browser

[javagl]

This has been on my TODO list for 7 years now. Let's promote it into an issue.

The jgltf-browser offers a menu to quickly load the official Khronos glTF sample models. Right now, this menu is defined in a home-baked sampleModels.config file that is loaded at startup.

All this was created waaay before the sample models repository had the structures that are necessary for automating that - namely, the model-index.json. (And of course, before the glTF-Sample-Models repo was moved to glTF-Sample-Assets...)

The browser should use the official model-index.json to fill the menus.

Yes, this will include glTF 1.0. I'm sticking to that.