Add workflow state to entity to be available in search_api for facets and fields

Attached is a patch that adds the workflow state name to the entity so it is available in search_api as a facet and field. In my case I'm using it with search_api_solr in a view, but it could be used for all back ends supported by search_api.

I'm thinking it's possible that you might want this in the core workflow module itself, but the code can be easily moved if necessary.