Simpler gene annotations in plotting functions

We typically have some marker information in the form of an Excel sheet, pandas DataFrame, and eventually a Python dictionary. Using these as gene annotations in various plotting functions (not pl.rank_genes_groups_* family but the others) is a very common task and it looks awesome thanks to @fidelram's var_group_* parameters.

It would be even more fantastic to be able to pass simple dict (e.g. the ones we already use in Malte's marker_gene_overlap) to plotting functions where var_group_positions and var_group_labels are populated automatically.

One caveat is that there might be genes covered by multiple keys, but this is similar to supplying overlapping var_group_positions in current api, which can exit with an error.

I already have a function for that but it's absolutely super ugly. I can send a PR after tidying it up, but if anyone else wants to do it, it's perfectly fine.

Start by inspecting the plotting functions that accept var_group_positions and var_group_labels, then compare their inputs with the marker_gene_overlap API. Done means a simple gene-to-group dictionary automatically populates the group positions and labels for the applicable plotting functions, while preserving the existing behavior for overlapping genes.