[feature request] Browse/edit model sub-meshes and their materials

[nightgryphon]

Most of loaded models consist of more than one material and more than one mesh but it is impossible to adjust them in inspector to find which changes to original model are required to fit the specific AFrame scene.
Having ability to browse meshes or at least materials inside the entity will make modeling for AFrame much easier.

[vincentfretin]

I also have a need to introspect a glb mainly to get the material name applied to a submesh, so I can use it with a color selector panel in my experiences (for example this scene with a car) using a material-values component to override some properties of material applied on a submesh at runtime.
Fixing the blend mode of a material would also be great, I had several glb I had to fix with blender to change the alpha blend mode.
That's probably something I'll implement in https://github.com/c-frame/aframe-editor
Doing a full glb editor and reexport to glb may be out of scope though, but I'm open to any PR later.