Have materials be able to be accessed independently of objects.

[atlawrie]

Requested feature below:

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It would be cool if I could load the BEER layers by clicking on the material on the right.

I'm still not sure how I can load the layers of a BEER material that is not assigned to an object, for now i guess i can just temporarily assign it to something.

Originally posted by @eballeste in https://github.com/bnpr/BEER/discussions/3#discussioncomment-1794391