We should load and index all available textures

[Argmaster]

Description

• Load and globally index all available item and block textures. Primary reason is to treat assets uniformly. Uniform indexing logic means simpler asset loading code.
• Move textures next to block/item definitions, instead of a nested textures hierarchy, similarly to how blueprints are handled. We could utilize extension like suffixed eg. .block.png .item.png etc. So the ID spaces are implicit.
• Inline item definitions in blocks use file names instead of IDs. This should be changed for consistency.

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I am primarily concerned with last of the issues, but I think other of the mentioned things can be relevant?