GLTF Mesh Import: Add material editor to model loader

[marchcat]

#4271 removed the call to the material editor when opening gltf/glb files, since the goal is to upload the textures directly, as is already done for Collada files.

The material loader is still available under "Build -> Upload -> Material".

This comment seems like a good idea:

Also allow a away to use previously uploaded materials that exist in inventory, to support update of mesh models without creating duplicates of any materials. Maybe by nominating a folder containing the materials to use if some means to partial match on material name or content deduplication can be done.

Originally posted by @aiaustin in #4204

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So the task is:

1. Improve the model loader UI so that its functionality extends beyond simply checking the binary "Upload Textures" option.
2. Update the UI to include a list of materials and the ability to select which ones should be uploaded with the model.
3. As a further enhancement, add the ability to modify these materials by applying other textures from disk or inventory.