Restore icons.toml configuration

[Sk7Str1p3]

Configuration with toml was way more human-friendly and easy, I think we should bring it back. Why would we even ditch it?

[AaronErhardt]

I think there was a strong technical reason. The toml file was super hard to locate, especially if cargo was configured to build crates in a centralized folder. If you find a solution to make it work without some weird hacks, we can go back.