If skins.js is going to be shared between maze & turtle now, then we should factor out common logic. At minimum, we shouldn't have the asymmetry in names: loadSkin and loadTurtleSkin.

The simplest thing to do: Just use a single common skins.load function and don't care if some fields aren't used by both apps.

A better option: Store `skin.root` and only common fields in skins.load, then add loadSkin functions to both maze & turtle. This way the unshared logic isn't loaded in to both apps & maintains proper separation.