Refine support for tables annotating dataframes

[LucaMarconato]

With #304, the color for categorical dataframe columns is specified via other dataframe columns, while the color for categorical obs is specified in the table (in .uns). Do we want to keep this behavior or to also allow for categorical dataframe columns to be annotated by tables?

I prefer to keep things like this because it maintains "data locality", but opening the conversation here.