Introducing term_metas

[hughwillfayle]

In pur work with WordPress we search for many solutions to have term metas. We found one which we can implement easily and it has been tested in some client projects. Based upon a plugin I'd like to contribute the API functions and the schematic to the core.

get_term_meta( $term_id, $key = '', $single = FALSE )

update_term_meta( $term_id, $meta_key, $meta_value, $prev_value = '' )

delete_term_meta_by_key( $term_meta_key )

get_term_custom( $term_id = 0 )

get_term_custom_keys( $term_id = 0 )

get_term_custom_values( $key = '', $term_id = 0 )

Removed unneccessary deletions

Replaced Doc Errors

Related: #19048, #10142

A "term_id" is not actually unique to a term in the general sense. It is unique to a single, unique name and slug pair. But it may exist in multiple taxonomies. That's the point of the term_taxonomy_id. So, "Apple" may be in the fruit taxonomy and the company taxonomy. But metadata stored against "Apple" doesn't make sense — you'll want to store it against "Apple" + fruit or "Apple" + company.

This is why our API passes around term_id + taxonomy to uniquely identify a term, as together they are synonymous to term_taxonomy_id.

This is a duplicate of #10142. There it is outlined why we have not done this yet, and what we'll need to do first to be able to do this in the future.