Change level select index sorting order

[SuperFromND]

Right now, Open Manifold's level list is quite simple: it indexes the folder path of every level in the levels folder, sorts it alphabetically, and... that's it!

This works fine enough, but I think it can be improved in a few different ways. I've created a couple different issues to lay out some improvement ideas I've had here and there.

Probably activated by pressing Triangle, this change would be pretty easy to implement as there's already a global level_index vector that can be re-sorted. All you'd have to do is use something like std::sort.

Alphabetically ascending and descending are trivial to implement this way, but more complicated sort orders (e.g. sorting by author name) would require indexing more data than just level paths, which we don't do right now.