Create an entity generic version of views_handler_field_term_node_tid

The new handler (views_handler_field_term_entity_tid? views_handler_field_entity_terms?) will work for all entity types and all backend types (Search API, SQL).

It should be based on the code from https://gist.github.com/3295371.

The patch should be submitted to Views.

After that, the handler should be added to the product variations view (the one that is embedded under "Quick Edit"), which will allow all relevant sizes and other attributes to show up for products (right now we show the color and the hat size).