generate thumbnails for imported 3D scenes

[ianmckenna87]

Outside of gridmap, there is simply no way to view 3d thumbnails of 3d models and assets. Thumbnails are generated for textures and for 3D scenes, but when working with gltf, fbx and the like, you're basically working blind when trying to place these files in a scene.

It would be a big improvement if these could be generated, even optionally at import and greatly speed up level design. As it stands, it's surely a pain point when working in 3D with Godot, and none of the plug-ins on the asset library address this issue elegantly enough.