Add an optional "icon" Tag to description

[earrgames]

I mentioned this before but not sure if everyone read or thought about it. Adding and optional tag outside the nft_object (in description) to define a base64 encoded icon (this format could be enforced to not create another tag to specify icon format...), the key could be called "b64pngicon" to avoid confusion or something. This is for archival/cache/gallery purpose, and previewing un-renderable NFTs by apps. the icon could be a 48x48 5 bits (32 colors indexed) [1.44 KB max] to not take much space or so.

Thanks ;)