Extract module: xmlsitemap_language

Problem/Motivation

Not all sites generate a sitemap.xml for each language. For example, I have a site which has subdomains like us.example.com, ca.example.com. The "context" for each sitemap is a domain record created with the "domain" module. Languages don't have any say over what appears in the sitemap.xml.

Steps to reproduce

Proposed resolution

Extract a module, say, xmlsitemap_language, which will provide the language_* hook implementations that currrently force the language context on each sitemap.

Remaining tasks

User interface changes

The "Language" column to be removed from the sitemap list page. Maybe, it can be replaced with a "Contexts" column which is populated dynamically with the context for each sitemap?

API changes

This is not compulsory, but a good to have. Say, create an XML Sitemap context plugin to allow managing things like Language and Domain context? Alternatively, depend on the Context module for this.

Data model changes

The "language" context becomes optional.