Editor: .gltf assets with utf8 characters in filenames can't be loaded

[hybridherbst]

While tackling various internationalization issues with glTF assets I made a few new test assets:

• Add URI test assets KhronosGroup/glTF-Sample-Assets#157

Almost none of them load at all when either the folder or the zip are dropped into modelviewer.dev/editor. Most of them load correctly in other glTF viewers, so something about path handling in the editor might not be quite correct.