Web: Support resources in dropped directories

[lexaknyazev]

Fixes #232.

Notes:

• Directory access is not available when the Validator is opened from a local disk, i.e., with the file:// URI. This is a Web platform limitation.
• Directories cannot be selected from the system dialog opened by the "Select" link. This is also a web platform limitation.
• Dropping a single directory with a glTF (or GLB) asset inside is intentionally not supported.

/cc @emackey @javagl @echadwick-artist

[javagl]

I'll have to read more in the linked issues and code changes, but maybe you can quickly confirm: With this change, it will be possible to drag-and-drop, for example, ...

• an example.gltf that refers to ./images/image.png
• together with that images subdirectory

and it will resolve that image as part of (or "in the context of") the dropped resources.

Is that correct?
(If so, then this does also resolve #208 ...)

[lexaknyazev]

This is correct.