Dynamic skybox list (editing UI)

[Jigoku]

I've just cobbled this together; a (potentially hackish) way of listing all the skyboxes in the editing UI, i'm sure it could be improved, but has the benefit of not needing to hardcode skybox paths/names.

Here's the patch if anyone wanted to look at it further;
https://gist.github.com/Jigoku/6c15195216322ee2aa5a8cbf2e408bbc

(diff based on 6767f32)

[lambda-11235]

What's wrong with the current skybox selection UI?

[no-lex]

It's entirely hardcoded.