feat: add new type of layer Categorized

[yohanboniface]

This is like Choropleth, but with categories instead of graduated.

fix #1433

I'd tried to keep only one class (Choropleth), but given Choropleth needs numbers while Categorized use anything and Choropleth needs breaks (range of values) while Categorized only checks for equality, this was ending with more mess.

Screencast from 2024-07-01 12-44-45.webm