Assign a domain to sitemaps of type index so only sitemaps with that domain are listed in the index

Problem/Motivation

I noticed that the sitemap domain selection only appear for sitemaps that generate URLs to entities but I have an index of sitemaps where all the sitemaps are listed for all the domains.

Steps to reproduce

Create a sitemap index, generate it and see all the sitemaps listed regardless of domain.

Proposed resolution

Add the domain selector also for sitemaps that have an index generator and use the setting to filter out sitemaps that do not belong to that domain.

Remaining tasks

Code

User interface changes

New selector for domain

API changes

Data model changes